regulator: dummy, make dummy_regulator_driver static

regulator: dummy, make dummy_regulator_driver static

When converting to faux_device the dummy_regulator_driver was
made non-static however it isn't exported or defined anywhere
outside the file it is in. Make it static to avoid the following
sparse warning:

drivers/regulator/dummy.c:59:24: warning: symbol 'dummy_regulator_driver' was not declared. Should it be static?

Fixes: dcd2a9a5550ef556c8 ("regulator: dummy: convert to use the faux device interface")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://patch.msgid.link/20260112154909.601987-1-ben.dooks@codethink.co.uk
Signed-off-by: Mark Brown <broonie@kernel.org>