regulator: Fix implicit use of notifier.h by driver.h

This was implicitly appearing by way of module.h -- but when
we fix that, we'll get this:

In file included from drivers/regulator/dummy.c:21:
include/linux/regulator/driver.h:197: error: field 'notifier' has incomplete type
make[3]: *** [drivers/regulator/dummy.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>