pull in stddef.h for NULL

This file uses NULL, but doesn't include headers for it.  It works
currently as the headers it does pull in happen to pull in the right
headers eventually.  Make it explicit to fix building with newer
kernel headers.

BUG=None
TEST=`emerge-x86-alex ply-image` still works

Change-Id: I7c1f7096370085956cd9e5f7a2cf9b7de72e5247
Reviewed-on: https://gerrit.chromium.org/gerrit/48857
Reviewed-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/src/ply-kms.c b/src/ply-kms.c
index 0093cb0..809514d 100644
--- a/src/ply-kms.c
+++ b/src/ply-kms.c
@@ -19,6 +19,7 @@
  *
  */
 
+#include <stddef.h>
 #include <xf86drm.h>
 #include <xf86drmMode.h>