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>

drivers/regulator/twl-regulator.c

@@ -10,6 +10,8 @@
  */
 
 #include <linux/module.h>
+#include <linux/string.h>
+#include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/err.h>
 #include <linux/platform_device.h>