pkg/tarheader: vet fix
pkg/tarheader: vet fix

Fix the following vet report:
  - pkg/tarheader/pop_darwin.go:1: +build comment must appear before package clause and be followed by a blank line

adfernandes/rkt,klizhentas/rkt,klizhentas/rkt,nhlfr/rkt,crawford/rkt,jzelinskie/rkt,achanda/rkt,crawford/rkt,ChengTiesheng/rkt,iankronquist/rkt,coreos/rkt,ramitsurana05/rkt,markllama/rkt,dgonyeo/rkt,clearlinux/rkt,MattDees/rkt,mike1729/rkt,krnowak/rkt,lsm5/rkt,Shashwatsh/rkt,lee212/rkt,tmrts/rkt,coreos/rkt,matthaias/rkt,sinfomicien/rkt,ybubnov/rkt,robszumski/rocket,coreos/rkt,il9ue/rkt,mjg59/rkt,xnox/rkt,jakon89/rkt,mattthias/pkg-rkt,squeed/rkt,lucasallan/rkt,jiachenning/rocket,iaguis/rkt,alban/rkt,shtouff/rkt,adfernandes/rkt,Wilybald/rkt,fdserr/rkt,joshix/rkt,Yhgenomics/rkt,cab105/rkt,monstermunchkin/rkt,linki/rkt,lee212/rkt,intfrr/rkt,sstabellini/rkt,bsmr-go/rkt,endocode/rkt,gdi2290/rocket,luebken/rkt,ChengTiesheng/rkt,liubq919/rkt,tv42/rkt,yifan-gu/rkt,jiachenning/rocket,shobhit85/rkt,rot26/rkt,michelangelo13/rkt,rkt/rkt,jjlakis/rkt,linki/rkt,tmrts/rkt,intelsdi-x/rkt,blixtra/rkt,squaremo/rkt,maquanyi/rkt,liugenping/rkt,squeed/rkt,michelangelo13/rkt,lsm5/rkt,kinvolk/rkt,tixxdz/rkt,wparad/rkt,luebken/rkt,jonboulle/rkt,tixxdz/rkt,euank/rkt,bsmr-go/rkt,marineam/rkt,dalanlan/rkt,mjg59/rkt,kinvolk/rkt,krnowak/rkt,iankronquist/rkt,joshix/rkt,ybubnov/rkt,ramitsurana05/rkt,MattDees/rkt,fdserr/rkt,yifan-gu/rocket,mike1729/rkt,intelsdi-x/rkt,shobhit85/rkt,marineam/rkt,cab105/rkt,kbrwn/rkt,eyakubovich/rkt,s-urbaniak/rkt,yifan-gu/rocket,alban/rkt,gdi2290/rocket,alban/rkt,endocode/rkt,mattthias/pkg-rkt,jjlakis/rkt,Wilybald/rkt,ramitsurana/rkt,sinfomicien/rkt,endocode/rkt,clearlinux/rkt,achanda/rkt,tixxdz/rkt,iaguis/rkt,crawford/rkt,yifan-gu/rocket,tv42/rkt,shtouff/rkt,jzelinskie/rkt,shobhit85/rkt,nhlfr/rkt,matthaias/rkt,sstabellini/rkt,akshaykarle/rkt,jonboulle/rkt,marineam/rkt,mjg59/rkt,kbrwn/rkt,robszumski/rocket,alexhersh/rkt,il9ue/rkt,liugenping/rkt,kinvolk/rkt,lsm5/rkt,smarvin/rkt,rkt/rkt,ramitsurana05/rkt,dalanlan/rkt,lucasallan/rkt,jasonchaffee/rkt,xnox/rkt,bramvdbogaerde/rocket-arm,rhvgoyal/rkt,sgotti/rkt,akshaykarle/rkt,xnox/rkt,dalanlan/rkt,blixtra/rkt,sstabellini/rkt,liugenping/rkt,fdserr/rkt,wparad/rkt,eyakubovich/rkt,MabinGo/rkt,smarvin/rkt,matthaias/rkt,s-urbaniak/rkt,adfernandes/rkt,maquanyi/rkt,Wilybald/rkt,bramvdbogaerde/rocket-arm,sgotti/rkt,shtouff/rkt,ramitsurana/rkt,ybubnov/rkt,robszumski/rocket,ChengTiesheng/rkt,rhvgoyal/rkt,Yhgenomics/rkt,chrisseto/rkt,jonboulle/rkt,mike1729/rkt,chrisseto/rkt,jasonchaffee/rkt,tv42/rkt,alexhersh/rkt,ramitsurana/rkt,michelangelo13/rkt,iaguis/rkt,kbrwn/rkt,rkt/rkt,jasonchaffee/rkt,intfrr/rkt,rhvgoyal/rkt,MabinGo/rkt,tmrts/rkt,dgonyeo/rkt,lucasallan/rkt,sinfomicien/rkt,s-urbaniak/rkt,liubq919/rkt,smarvin/rkt,MabinGo/rkt,sgotti/rkt,il9ue/rkt,akshaykarle/rkt,rot26/rkt,blixtra/rkt,luebken/rkt,lee212/rkt,jzelinskie/rkt,maquanyi/rkt,yifan-gu/rkt,markllama/rkt,squaremo/rkt,krnowak/rkt,euank/rkt,linki/rkt,jjlakis/rkt,alexhersh/rkt,monstermunchkin/rkt,Wilybald/rkt,clearlinux/rkt,chrisseto/rkt,Shashwatsh/rkt,eyakubovich/rkt,dgonyeo/rkt,liubq919/rkt,Yhgenomics/rkt,squeed/rkt,achanda/rkt,mattthias/pkg-rkt,MattDees/rkt,lucasallan/rkt,nhlfr/rkt,iankronquist/rkt,euank/rkt,cab105/rkt,yifan-gu/rkt,Shashwatsh/rkt,jakon89/rkt,intelsdi-x/rkt,kinvolk/rkt,rot26/rkt,joshix/rkt,squaremo/rkt,joshix/rkt,monstermunchkin/rkt,klizhentas/rkt,intfrr/rkt,rkt/rkt,jakon89/rkt,wparad/rkt,bsmr-go/rkt,markllama/rkt