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.
Noticed while checking whether there were other issues similar to #3564 (closed).