Fix missing stdlib includes in xf86drmMode.h

Including xf86drmMode.h results in undefined references to uint32_t
and ssize_t. Include the stdlib headers that define them to allow the
file to be included without xf86drm.h.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

xf86drmMode.h

@@ -41,6 +41,8 @@ extern "C" {
 #endif
 
 #include <drm.h>
+#include <stddef.h>
+#include <stdint.h>
 
 /*
  * This is the interface for modesetting for drm.