Fix folly/FBString.h build error
Fix folly/FBString.h build error

Summary:
Looks like D880140 left off this part when it was committed.

This fixes the build.

Test Plan: runtests

Reviewed By: vini@fb.com

FB internal diff: D881445

SeanRBurton/folly,alexst07/folly,lifei/folly,tomhughes/folly,xzmagic/folly,xzmagic/folly,unmeshvrije/C--,juniway/folly,fw1121/folly,arg0/folly,bsampath/folly,stonegithubs/folly,leolujuyi/folly,doctaweeks/folly,theiver9827/folly,rklabs/folly,clearlylin/folly,SammyK/folly,guker/folly,shaobz/folly,doctaweeks/folly,ykarlsbrun/folly,SammyK/folly,kernelim/folly,Orvid/folly,ykarlsbrun/folly,Eagle-X/folly,renyinew/folly,Orvid/folly,nickhen/folly,arg0/folly,guker/folly,romange/folly,shaobz/folly,yangjin-unique/folly,clearlinux/folly,PPC64/folly,romange/folly,PPC64/folly,tempbottle/folly,sakishum/folly,gaoyingie/folly,floxard/folly,zhiweicai/folly,doctaweeks/folly,SeanRBurton/folly,tempbottle/folly,stonegithubs/folly,tomhughes/folly,loversInJapan/folly,raphaelamorim/folly,romange/folly,reddit/folly,alexst07/folly,rklabs/folly,cole14/folly,nickhen/folly,stonegithubs/folly,floxard/folly,brunomorishita/folly,charsyam/folly,Orvid/folly,leolujuyi/folly,romange/folly,bowlofstew/folly,stonegithubs/folly,loverszhaokai/folly,raphaelamorim/folly,PPC64/folly,zhiweicai/folly,mqeizi/folly,brunomorishita/folly,KSreeHarsha/folly,unmeshvrije/C--,loversInJapan/folly,brunomorishita/folly,zhiweicai/folly,clearlylin/folly,arg0/folly,loverszhaokai/folly,SammyK/folly,upsoft/folly,lifei/folly,doctaweeks/folly,fw1121/folly,constantine001/folly,arg0/folly,rklabs/folly,bobegir/folly,hongliangzhao/folly,Hincoin/folly,bikong2/folly,wildinto/folly,arg0/folly,kernelim/folly,bobegir/folly,charsyam/folly,kernelim/folly,bobegir/folly,nickhen/folly,upsoft/folly,project-zerus/folly,bowlofstew/folly,wildinto/folly,CJstar/folly,fw1121/folly,Eagle-X/folly,wildinto/folly,tomhughes/folly,xzmagic/folly,renyinew/folly,unmeshvrije/C--,wildinto/folly,nickhen/folly,hongliangzhao/folly,colemancda/folly,project-zerus/folly,lifei/folly,raphaelamorim/folly,gavioto/folly,colemancda/folly,chjp2046/folly,colemancda/folly,facebook/folly,bsampath/folly,colemancda/folly,fw1121/folly,PoisonBOx/folly,clearlinux/folly,facebook/folly,CJstar/folly,upsoft/folly,facebook/folly,Eagle-X/folly,loverszhaokai/folly,reddit/folly,loverszhaokai/folly,juniway/folly,loversInJapan/folly,gavioto/folly,bowlofstew/folly,PoisonBOx/folly,theiver9827/folly,guker/folly,clearlylin/folly,unmeshvrije/C--,leolujuyi/folly,tomhughes/folly,gavioto/folly,sonnyhu/folly,Hincoin/folly,gavioto/folly,fw1121/folly,alexst07/folly,Eagle-X/folly,SammyK/folly,constantine001/folly,SammyK/folly,rklabs/folly,sonnyhu/folly,facebook/folly,tomhughes/folly,Hincoin/folly,floxard/folly,bowlofstew/folly,rklabs/folly,charsyam/folly,reddit/folly,clearlinux/folly,constantine001/folly,cppfool/folly,PPC64/folly,mqeizi/folly,PPC64/folly,renyinew/folly,sakishum/folly,facebook/folly,clearlinux/folly,CJstar/folly,leolujuyi/folly,SeanRBurton/folly,renyinew/folly,loverszhaokai/folly,leolujuyi/folly,juniway/folly,xzmagic/folly,hongliangzhao/folly,constantine001/folly,kernelim/folly,Orvid/folly,bsampath/folly,hongliangzhao/folly,juniway/folly,tempbottle/folly,cppfool/folly,raphaelamorim/folly,gavioto/folly,lifei/folly,shaobz/folly,Hincoin/folly,bsampath/folly,KSreeHarsha/folly,sonnyhu/folly,CJstar/folly,floxard/folly,shaobz/folly,colemancda/folly,sakishum/folly,ykarlsbrun/folly,cppfool/folly,unmeshvrije/C--,chjp2046/folly,sonnyhu/folly,Orvid/folly,SeanRBurton/folly,yangjin-unique/folly,unmeshvrije/C--,cppfool/folly,project-zerus/folly,doctaweeks/folly,theiver9827/folly,cole14/folly,gaoyingie/folly,clearlylin/folly,raphaelamorim/folly,KSreeHarsha/folly,bikong2/folly,chjp2046/folly,bobegir/folly,cole14/folly,alexst07/folly,SeanRBurton/folly,sonnyhu/folly,bobegir/folly,zhiweicai/folly,project-zerus/folly,brunomorishita/folly,upsoft/folly,nickhen/folly,PoisonBOx/folly,KSreeHarsha/folly,clearlylin/folly,theiver9827/folly,KSreeHarsha/folly,gaoyingie/folly,cole14/folly,Eagle-X/folly,alexst07/folly,shaobz/folly,cole14/folly,xzmagic/folly,guker/folly,bikong2/folly,juniway/folly,floxard/folly,hongliangzhao/folly,tempbottle/folly,lifei/folly,PoisonBOx/folly,constantine001/folly,theiver9827/folly,reddit/folly,bikong2/folly,stonegithubs/folly,zhiweicai/folly,kernelim/folly,ykarlsbrun/folly,project-zerus/folly,guker/folly,gaoyingie/folly,Hincoin/folly,reddit/folly,brunomorishita/folly,cppfool/folly,sakishum/folly,mqeizi/folly,loversInJapan/folly,bsampath/folly,yangjin-unique/folly,charsyam/folly,renyinew/folly,wildinto/folly,upsoft/folly,yangjin-unique/folly,tempbottle/folly,romange/folly,mqeizi/folly,sakishum/folly,loversInJapan/folly,yangjin-unique/folly,ykarlsbrun/folly,mqeizi/folly,chjp2046/folly,chjp2046/folly,PoisonBOx/folly,clearlinux/folly,bowlofstew/folly,bikong2/folly,CJstar/folly,gaoyingie/folly