build-sys: fix out-of-tree build of vapi
build-sys: fix out-of-tree build of vapi

Reference custom file by path.

https://bugzilla.gnome.org/show_bug.cgi?id=741315

jasuarez/grilo,grilofw/grilo,jasuarez/grilo,grilofw/grilo,MathieuDuponchelle/grilo,MathieuDuponchelle/grilo,grilofw/grilo,MathieuDuponchelle/grilo,GNOME/grilo,GNOME/grilo,jasuarez/grilo