regmap: Add missing header for !CONFIG_REGMAP stubs

regmap: Add missing header for !CONFIG_REGMAP stubs

regmap.h requires linux/err.h if CONFIG_REGMAP is not defined. Without it I get
error.
CC      drivers/media/platform/exynos4-is/fimc-reg.o
In file included from drivers/media/platform/exynos4-is/fimc-reg.c:14:0:
include/linux/regmap.h: In function ‘regmap_write’:
include/linux/regmap.h:525:10: error: ‘EINVAL’ undeclared (first use in this function)
include/linux/regmap.h:525:10: note: each undeclared identifier is reported only once for each function it appears in

Signed-off-by: Mateusz Krawczuk <m.krawczuk@partner.samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@kernel.org