build-time paths to gio/gunixmounts.h, etc. end up in Gio-2.0.gir, GioUnix-2.0.gir

To reproduce: build GLib 2.83.2 with introspection enabled, on Debian unstable. Install it into a temporary location and inspect the installed Gio-2.0.gir, GioUnix-2.0.gir.

Expected result: <c:include name="gio/gunixmounts.h"/>, etc.

Actual result: <c:include name="/build/glib-2.0-XXXXXX/glib2.0-2.83.2/gio/gunixmounts.h"/>, etc., where /build/glib-2.0-XXXXXX/glib2.0-2.83.2 was the directory used by Debian build infrastructure

I think this is a regression caused by 51e3e7d9. We specifically do not want the full_path() to these files: we want their path relative to the root.