gdbus-codegen: Call abspath() earlier

The previous change from https://bugzilla.gnome.org/show_bug.cgi?id=786785
didn't actually work (for me at least) in the `/bin/gdbus-codegen` case
as the relative `/bin/../share` path works, but then `os.path.abspath()`
breaks it (as it's not doing `realpath()`, and let's not go there).

Fix this by doing the `abspath` first.

Downstream: https://bugzilla.redhat.com/show_bug.cgi?id=1507661

https://bugzilla.gnome.org/show_bug.cgi?id=789723