Final excision of libgsystem dependency

Lots and lots of preparation led to this moment - when nothing
apparent changes for users!  Woo!

But seriously, having the extra dependency is a minor annoyance, and
in the big picture I think the libgsystem idea was wrong - we need to
land things in GLib, and use git submodules for API-unstable or
Linux-specific sharing.  For a lot of OSTree, the libgsystem `GFile*`
orientation was also wrong, we really want fd-relative.

Closes: #444
Approved by: jlebon

src/libotutil/ot-gio-utils.h

@@ -115,4 +115,14 @@ ot_file_enumerator_iterate (GFileEnumerator  *direnum,
 #endif
 #define g_file_enumerator_iterate ot_file_enumerator_iterate
 
+const char *
+ot_file_get_path_cached (GFile *file);
+
+static inline
+const char *
+gs_file_get_path_cached (GFile *file)
+{
+  return ot_file_get_path_cached (file);
+}
+
 G_END_DECLS