regulator: stw481x: compile on COMPILE_TEST

The driver depends on MFD_STW481X but there isn't a build dependency so
it's a good idea to allow the driver to always be built when the
COMPILE_TEST option is enabled.

That way, the driver can be built with a config generated by make
allyesconfig and check if a patch would break the build.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>