[cmake] Make paths of headers when building G__Core dictionary relative.
[cmake] Make paths of headers when building G__Core dictionary relative.

Core is split differently than the rest of ROOT. We generate the target of
G__Core from within core/base, thus CMAKE_CURRENT_SOURCE_DIR cannot make
headers from the rest of core subfolders relative.

gbitzes/root,mhuwiler/rootauto,davidlt/root,agarciamontoro/root,mhuwiler/rootauto,agarciamontoro/root,beniz/root,abhinavmoudgil95/root,davidlt/root,bbockelm/root,simonpf/root,gbitzes/root,davidlt/root,bbockelm/root,karies/root,beniz/root,bbockelm/root,simonpf/root,buuck/root,agarciamontoro/root,buuck/root,beniz/root,bbockelm/root,olifre/root,zzxuanyuan/root-compressor-dummy,zzxuanyuan/root-compressor-dummy,abhinavmoudgil95/root,zzxuanyuan/root,root-mirror/root,karies/root,satyarth934/root,beniz/root,gganis/root,karies/root,bbockelm/root,satyarth934/root,mhuwiler/rootauto,satyarth934/root,satyarth934/root,karies/root,simonpf/root,buuck/root,davidlt/root,agarciamontoro/root,abhinavmoudgil95/root,mhuwiler/rootauto,gganis/root,davidlt/root,mhuwiler/rootauto,BerserkerTroll/root,BerserkerTroll/root,gbitzes/root,mhuwiler/rootauto,bbockelm/root,agarciamontoro/root,olifre/root,agarciamontoro/root,root-mirror/root,root-mirror/root,beniz/root,abhinavmoudgil95/root,zzxuanyuan/root,olifre/root,mhuwiler/rootauto,gganis/root,buuck/root,BerserkerTroll/root,zzxuanyuan/root,root-mirror/root,zzxuanyuan/root,buuck/root,olifre/root,davidlt/root,gbitzes/root,davidlt/root,zzxuanyuan/root,karies/root,gbitzes/root,zzxuanyuan/root,root-mirror/root,abhinavmoudgil95/root,root-mirror/root,zzxuanyuan/root,beniz/root,buuck/root,gganis/root,simonpf/root,BerserkerTroll/root,karies/root,agarciamontoro/root,zzxuanyuan/root,zzxuanyuan/root,zzxuanyuan/root-compressor-dummy,root-mirror/root,zzxuanyuan/root-compressor-dummy,gganis/root,root-mirror/root,satyarth934/root,abhinavmoudgil95/root,BerserkerTroll/root,olifre/root,bbockelm/root,zzxuanyuan/root-compressor-dummy,buuck/root,zzxuanyuan/root-compressor-dummy,satyarth934/root,BerserkerTroll/root,olifre/root,gbitzes/root,buuck/root,zzxuanyuan/root,beniz/root,gganis/root,bbockelm/root,abhinavmoudgil95/root,olifre/root,bbockelm/root,BerserkerTroll/root,zzxuanyuan/root-compressor-dummy,beniz/root,buuck/root,mhuwiler/rootauto,simonpf/root,zzxuanyuan/root-compressor-dummy,agarciamontoro/root,simonpf/root,bbockelm/root,abhinavmoudgil95/root,abhinavmoudgil95/root,abhinavmoudgil95/root,simonpf/root,simonpf/root,gbitzes/root,zzxuanyuan/root-compressor-dummy,karies/root,gganis/root,olifre/root,karies/root,simonpf/root,karies/root,karies/root,olifre/root,bbockelm/root,agarciamontoro/root,davidlt/root,olifre/root,zzxuanyuan/root,BerserkerTroll/root,gbitzes/root,root-mirror/root,buuck/root,zzxuanyuan/root-compressor-dummy,gganis/root,davidlt/root,satyarth934/root,buuck/root,davidlt/root,gbitzes/root,beniz/root,beniz/root,gganis/root,BerserkerTroll/root,root-mirror/root,gbitzes/root,gganis/root,karies/root,agarciamontoro/root,gganis/root,simonpf/root,BerserkerTroll/root,mhuwiler/rootauto,satyarth934/root,zzxuanyuan/root-compressor-dummy,zzxuanyuan/root,gbitzes/root,mhuwiler/rootauto,agarciamontoro/root,mhuwiler/rootauto,satyarth934/root,root-mirror/root,davidlt/root,BerserkerTroll/root,abhinavmoudgil95/root,beniz/root,satyarth934/root,satyarth934/root,simonpf/root,olifre/root