gio, gobject: Improve reproducibility of enumtypes headers

`@filename@` expands to the (absolute or relative) path from the
build directory to the source directory, which can be rather verbose.
In practice Meson usually (always?) generates a relative path, but
even so, the resulting installed header is not necessarily reproducible
if using different build directories outside the source directory.

We don't really need a full path here anyway: the basename is enough
of a hint to point a reader towards the file where the underlying
enum was defined.

Signed-off-by: Simon McVittie <smcv@collabora.com>

gobject/glib-enumtypes.h.template

@@ -9,7 +9,7 @@ G_BEGIN_DECLS
 
 /*** BEGIN file-production ***/
 
-/* enumerations from "@filename@" */
+/* enumerations from "@basename@" */
 /*** END file-production ***/
 
 /*** BEGIN value-header ***/