gallium/aux/os/os_thread.h: Silence -Wunused-param.

With --disable-debug a parameter is not used. Silence this
warning by fake-using it.

Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/os/os_thread.h

@@ -56,6 +56,8 @@ __pipe_mutex_assert_locked(mtx_t *mutex)
    assert(ret == thrd_busy);
    if (ret == thrd_success)
       mtx_unlock(mutex);
+#else
+   (void)mutex;
 #endif
 }