gpio: sim: include a missing header

We're using various ERR macros from linux/err.h but the include is
missing. Add it.

Fixes: cb8c474e ("gpio: sim: new testing module")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpio-sim.c

@@ -12,6 +12,7 @@
 #include <linux/completion.h>
 #include <linux/configfs.h>
 #include <linux/device.h>
+#include <linux/err.h>
 #include <linux/gpio/consumer.h>
 #include <linux/gpio/driver.h>
 #include <linux/gpio/machine.h>