tests: Cleanup superfluous headers

Mostly remove headers that aren't actually needed for anything.
Add stdint.h to permit dropping xf86drm.h, which is otherwise unneeded.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Acked-by: Marek Chalupa <mchqwerty@gmail.com>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/tests/setbacklight.c b/tests/setbacklight.c
index c8eba43..5c79b90 100644
--- a/tests/setbacklight.c
+++ b/tests/setbacklight.c
@@ -32,9 +32,9 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdint.h>
 #include <unistd.h>
 #include <fcntl.h>
-#include <xf86drm.h>
 #include <xf86drmMode.h>
 
 #include "libbacklight.h"