st/mesa: Destroy buffer object's mutex.

Ideally we should have a _mesa_cleanup_buffer_object function in
src/mesa/bufferobj.c so that the destruction logic resided in a single
place.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/state_tracker/st_cb_bufferobjects.c

@@ -83,6 +83,7 @@ st_bufferobj_free(struct gl_context *ctx, struct gl_buffer_object *obj)
    if (st_obj->buffer)
       pipe_resource_reference(&st_obj->buffer, NULL);
 
+   mtx_destroy(&st_obj->Base.Mutex);
    free(st_obj->Base.Label);
    free(st_obj);
 }