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>

src/gallium/state_trackers/xvmc/Makefile.am

@@ -27,6 +27,7 @@ AM_CFLAGS = \
 	$(GALLIUM_CFLAGS) \
 	$(VISIBILITY_CFLAGS) \
 	$(VL_CFLAGS) \
+	$(X11_INCLUDES) \
 	$(XCB_DRI3_CFLAGS) \
 	$(XVMC_CFLAGS)