regulator: core: Fix modpost error "regulator_get_regmap" undefined

Fix the modpost error "regulator_get_regmap" undefined by adding export
symbol.

Fixes: 04eca28c

 ("regulator: Add helpers for low-level register access")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406110117.mk5UR3VZ-lkp@intel.com
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20240610195532.175942-1-biju.das.jz@bp.renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>