Hide clutter_context_get_default()

The clutter_context_get_default() function is private, but shared
across Clutter. For this reason, it should be prefixed by '_' so
that the symbol is hidden from the shared object.

clutter/sdl/clutter-event-sdl.c

@@ -335,7 +335,7 @@ clutter_event_dispatch (GSource     *source,
 
   clutter_threads_enter ();
 
-  clutter_context = clutter_context_get_default ();
+  clutter_context = _clutter_context_get_default ();
 
   while (SDL_PollEvent(&sdl_event))
     {