vs2010: fix relative path to target private dir for generators
vs2010: fix relative path to target private dir for generators

backend.get_target_private_dir() includes the target directory in the path.
However, we want to treat all paths relative from the target directory,
because that's where our VS project file lives in.

aaronp24/meson,jeandet/meson,jeandet/meson,ernestask/meson,pexip/meson,centricular/meson,trhd/meson,ernestask/meson,jpakkane/meson,jeandet/meson,thiblahute/meson,MathieuDuponchelle/meson,wberrier/meson,ernestask/meson,rhd/meson,centricular/meson,aaronp24/meson,MathieuDuponchelle/meson,QuLogic/meson,rhd/meson,aaronp24/meson,jeandet/meson,fmuellner/meson,trhd/meson,wberrier/meson,trhd/meson,thiblahute/meson,ernestask/meson,aaronp24/meson,trhd/meson,jpakkane/meson,aaronp24/meson,thiblahute/meson,wberrier/meson,trhd/meson,centricular/meson,fmuellner/meson,aaronp24/meson,QuLogic/meson,mesonbuild/meson,fmuellner/meson,thiblahute/meson,trhd/meson,pexip/meson,wberrier/meson,mesonbuild/meson,pexip/meson,thiblahute/meson,thiblahute/meson,fmuellner/meson,pexip/meson,wberrier/meson,centricular/meson,pexip/meson,ernestask/meson,jeandet/meson,germandiagogomez/meson,pexip/meson,MathieuDuponchelle/meson,QuLogic/meson,trhd/meson,aaronp24/meson,mesonbuild/meson,wberrier/meson,ernestask/meson,mesonbuild/meson,MathieuDuponchelle/meson,germandiagogomez/meson,thiblahute/meson,becm/meson,aaronp24/meson,jpakkane/meson,MathieuDuponchelle/meson,QuLogic/meson,mesonbuild/meson,rhd/meson,jpakkane/meson,becm/meson,becm/meson,ernestask/meson,jpakkane/meson,QuLogic/meson,fmuellner/meson,rhd/meson,fmuellner/meson,jpakkane/meson,centricular/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,rhd/meson,QuLogic/meson,thiblahute/meson,becm/meson,mesonbuild/meson,aaronp24/meson,fmuellner/meson,QuLogic/meson,becm/meson,becm/meson,centricular/meson,wberrier/meson,jeandet/meson,QuLogic/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,centricular/meson,fmuellner/meson,germandiagogomez/meson,ernestask/meson,MathieuDuponchelle/meson,wberrier/meson,centricular/meson,becm/meson,pexip/meson,QuLogic/meson,thiblahute/meson,rhd/meson,germandiagogomez/meson,ernestask/meson,trhd/meson,becm/meson,germandiagogomez/meson,jeandet/meson,rhd/meson,mesonbuild/meson,becm/meson,rhd/meson,pexip/meson,pexip/meson,trhd/meson,jeandet/meson,fmuellner/meson,germandiagogomez/meson,germandiagogomez/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,jeandet/meson