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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib-of.c

@@ -16,6 +16,7 @@
 #include <linux/errno.h>
 #include <linux/module.h>
 #include <linux/io.h>
+#include <linux/io-mapping.h>
 #include <linux/gpio/consumer.h>
 #include <linux/of.h>
 #include <linux/of_address.h>