drm/xe/kunit: fix link failure with built-in xe

drm/xe/kunit: fix link failure with built-in xe

When the driver is built-in but the tests are in loadable modules,
the helpers don't actually get put into the driver:

ERROR: modpost: "xe_kunit_helper_alloc_xe_device" [drivers/gpu/drm/xe/tests/xe_test.ko] undefined!

Change the Makefile to ensure they are always part of the driver
even when the rest of the kunit tests are in loadable modules.

Fixes: 5095d13d758b ("drm/xe/kunit: Define helper functions to allocate fake xe device")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240226124736.1272949-1-arnd@kernel.org
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>