vout: silence unused warning

The vout parameter of vout_CloseWrapper() is only used on Windows, causing a warning on other platforms:

../../src/video_output/vout_wrapper.c:131:39: warning: unused parameter ‘vout’ [-Wunused-parameter]
  131 | void vout_CloseWrapper(vout_thread_t *vout, vout_thread_private_t *sys, vout_display_t *vd)
      |                        ~~~~~~~~~~~~~~~^~~~