Include XvMClib.h
Include XvMClib.h

This fixes compilation breakage after 9190399907610486001f3676945958dbab4b837a in videoproto

Found by tinderbox

Signed-off-by: Jeremy Huddleston <db3cec0cba09acb9cf8fd8bd399a3b27498cf883@freedesktop.org>

freedesktop-unofficial-mirror/xorg__lib__libXvMC,freedesktop-unofficial-mirror/xorg__lib__libXvMC,freedesktop-unofficial-mirror/xorg__lib__libXvMC