regmap: Add another missing header for !CONFIG_REGMAP stubs

The use of WARN_ON() needs the definitions from bug.h, without it
you can get:

include/linux/regmap.h: In function 'regmap_write':
include/linux/regmap.h:525:2: error: implicit declaration of function 'WARN_ONCE' [-Werror=implicit-function-declaration]

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org