XQuartz: Ensure <dix-config.h> is included in capabilities.c

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>

hw/xquartz/GL/capabilities.c

@@ -20,6 +20,10 @@
  * DEALINGS IN THE SOFTWARE.
  */
 
+#ifdef HAVE_DIX_CONFIG_H
+#include <dix-config.h>
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <assert.h>