lib: unnecessary header removal for drmtest.h, part 2

I've left unistd.h in it - it's not strictly required but most users
of drmtest.h want it for the open helpers, and then you kinda need to
close that file descriptor again ...

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

lib/igt_kms.h

@@ -29,6 +29,8 @@
 #include <stdint.h>
 #include <cairo.h>
 
+#include <xf86drmMode.h>
+
 #include "igt_display.h"
 
 struct kmstest_connector_config {