drm/tests: helpers: Add missing export

drm_kunit_device_init() is a public function meant to be used by other
tests, but isn't exported. This leads to modpost errors when the other
tests are compiled as module.
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221128081938.742410-3-maxime@cerno.tech

drivers/gpu/drm/tests/drm_kunit_helpers.c

@@ -66,6 +66,7 @@ struct drm_device *drm_kunit_device_init(struct kunit *test, u32 features, char
 
 	return drm;
 }
+EXPORT_SYMBOL(drm_kunit_device_init);
 
 MODULE_AUTHOR("Maxime Ripard <maxime@cerno.tech>");
 MODULE_LICENSE("GPL");