drm/tests: Include helpers header

The kunit helpers code weren't including its header, leading to a
warning that no previous prototype had been defined for public
functions.

Include the matching header to fix the warning.

Fixes: 44a3928324 ("drm/tests: Add Kunit Helpers")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Link: https://lore.kernel.org/r/20221116091712.1309651-1-maxime@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/tests/drm_kunit_helpers.c

@@ -5,6 +5,8 @@
 
 #include <linux/device.h>
 
+#include "drm_kunit_helpers.h"
+
 struct kunit_dev {
 	struct drm_device base;
 };