build-sys: fix gio-2.0 gir bindings errors

Use vala gio-2.0 definition to avoid the gir error:

Gio-2.0.gir:79247.7-79251.24: error: `UnixSocketAddress' already
contains a definition for `abstract'
Gio-2.0.gir:79093.7-79095.21: note: previous definition of `abstract'
was here

https://bugzilla.gnome.org/show_bug.cgi?id=741315
build-sys: fix out-of-tree build of vapi

Reference custom file by path.

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