st/xvmc: Add X11 include path.

This patch fixes this build error.

  CC       tests/xvmc_bench.o
In file included from tests/xvmc_bench.c:35:
tests/testlib.h:38:10: fatal error: 'X11/Xlib.h' file not found
         ^~~~~~~~~~~~

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 4f74580d)