drm/xe: Fix compilation without CONFIG_KUNIT

It looks that declaration of kunit_get_current_test() was
available in xe_guc_relay.c only with CONFIG_KUNIT enabled.
If CONFIG_KUNIT is disabled we fail with:

In file included from ../include/linux/build_bug.h:5,
                 from ../include/linux/bitfield.h:10,
                 from ../drivers/gpu/drm/xe/xe_guc_relay.c:6:
../drivers/gpu/drm/xe/xe_guc_relay.c: In function ‘xe_guc_relay_process_guc2vf’:
../drivers/gpu/drm/xe/xe_guc_relay.c:863:52: error: implicit declaration of function ‘kunit_get_current_test’ [-Werror=implicit-function-declaration]
  863 |  if (unlikely(!IS_SRIOV_VF(relay_to_xe(relay)) && !kunit_get_current_test()))
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~
../include/linux/compiler.h:77:42: note: in definition of macro ‘unlikely’
   77 | # define unlikely(x) __builtin_expect(!!(x), 0)
      |                                          ^

Reported-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Fixes: 811fe9f5

 ("drm/xe/guc: Introduce Relay Communication for SR-IOV")
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Tested-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://lore.kernel.org/r/20240105171947.321-1-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>