gpio: include <linux/io-mapping.h> in gpiolib-of

When enabling the gpiolib for all archs a build robot came
up with this:

All errors (new ones prefixed by >>):

   drivers/gpio/gpiolib-of.c: In function 'of_mm_gpiochip_add_data':
>> drivers/gpio/gpiolib-of.c:317:2: error: implicit declaration of
   function 'iounmap' [-Werror=implicit-function-declaration]
     iounmap(mm_gc->regs);
     ^~~~~~~
   cc1: some warnings being treated as errors

Fix this by including <linux/io-mapping.h> explicitly.

Fixes: 296ad4ac

 ("gpio: remove deps on ARCH_[WANT_OPTIONAL|REQUIRE]_GPIOLIB")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>