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>

drivers/regulator/Kconfig

@@ -627,7 +627,7 @@ config REGULATOR_TI_ABB
 
 config REGULATOR_STW481X_VMMC
 	bool "ST Microelectronics STW481X VMMC regulator"
-	depends on MFD_STW481X
+	depends on MFD_STW481X || COMPILE_TEST
 	default y if MFD_STW481X
 	help
 	  This driver supports the internal VMMC regulator in the STw481x