build: Add missing "static" keyword where it should be used

Otherwise we fail to build with -Werror=missing-prototypes.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

gio/tests/gdbus-peer.c

@@ -1682,7 +1682,7 @@ codegen_service_thread_func (gpointer user_data)
 }
 
 
-gboolean
+static gboolean
 codegen_quit_mainloop_timeout (gpointer data)
 {
   g_main_loop_quit (loop);