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: 44a3928324e9 ("drm/tests: Add Kunit Helpers")
Reported-by: kernel test robot <[e-mail]>
Reviewed-by: Maíra Canal <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Maxime Ripard <[e-mail]>

drivers/gpu/drm/tests/drm_kunit_helpers.c

···
 
 #include <linux/device.h>
 
+#include "drm_kunit_helpers.h"
+
 struct kunit_dev {
 	struct drm_device base;
 };