isl_list.h: allow inclusion from C++
isl_list.h: allow inclusion from C++

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

serge-sans-paille/isl,jleben/isl,simbuerg/isl,VanirLLVM/toolchain_isl,KangDroidSMProject/ISL,cfx-next/toolchain_isl-upstream,crossbuild/isl,Distrotech/isl,nicolasvasilache/isl,tobig/isl,PollyLabs/isl,inducer/isl-mirror,BobSaget-Mod/libisl,SaberMod/isl-current,crossbuild/isl,serge-sans-paille/isl,jleben/isl,Meinersbur/isl,UBERTC/isl,simbuerg/isl,PollyLabs/isl,BenzoSM/isl,nicolasvasilache/isl,KangDroidSMProject/ISL,crossbuild/isl,crossbuild/isl,VanirLLVM/toolchain_isl,abduld/isl,pierrotdelalune/isl,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,BenzoSM/isl,tobig/isl,UBERTC/isl,evaautomation/isl,BobSaget-Mod/libisl,Distrotech/isl,pierrotdelalune/isl,BenzoSM/isl,SaberMod/isl-current,nicolasvasilache/isl,pierrotdelalune/isl,SaberMod/isl-current,Meinersbur/isl,PollyLabs/isl,KangDroidSMProject/ISL,VanirLLVM/toolchain_isl,epowers/isl,serge-sans-paille/isl,epowers/isl,pierrotdelalune/isl,abduld/isl,tobig/isl,evaautomation/isl,BobSaget-Mod/libisl,pierrotdelalune/isl,BenzoSM/isl,epowers/isl,abduld/isl,BobSaget-Mod/libisl,jleben/isl,Meinersbur/isl,Distrotech/isl,Meinersbur/isl,evaautomation/isl,BenzoSM/isl,UBERTC/isl,KangDroidSMProject/ISL,Distrotech/isl,cfx-next/toolchain_isl-upstream,cfx-next/toolchain_isl-upstream,UBERTC/isl,cfx-next/toolchain_isl-upstream,KangDroidSMProject/ISL,Distrotech/isl,simbuerg/isl,inducer/isl-mirror,VanirLLVM/toolchain_isl,serge-sans-paille/isl,cfx-next/toolchain_isl-upstream,evaautomation/isl,abduld/isl,jleben/isl,tobig/isl,simbuerg/isl,nicolasvasilache/isl,BobSaget-Mod/libisl,VanirLLVM/toolchain_isl,SaberMod/isl-current,jleben/isl,simbuerg/isl,inducer/isl-mirror,inducer/isl-mirror,PollyLabs/isl,epowers/isl,epowers/isl,serge-sans-paille/isl