gbm: add a couple missing includes

Needed for memset() and drmIoctl().

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>

src/gbm/backends/dri/gbm_driint.h

@@ -28,6 +28,8 @@
 #ifndef _GBM_DRI_INTERNAL_H_
 #define _GBM_DRI_INTERNAL_H_
 
+#include <xf86drm.h>
+#include <string.h>
 #include <sys/mman.h>
 #include "gbmint.h"
 #include "c11/threads.h"