media: staging/imx: Allow driver to build if COMPILE_TEST is enabled

The driver has runtime but no build time dependency with IMX_IPUV3_CORE,
so can be built for testing purposes if COMPILE_TEST option is enabled.

This is useful to have more build coverage and make sure that the driver
is not affected by changes that could cause build regressions.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>