regulator: using kfree() requires including slab.h

Fixes this build error:

drivers/regulator/twl-regulator.c: In function 'twlreg_probe':
drivers/regulator/twl-regulator.c:1229:3: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]

Also added string.h for kmemdup().

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>