gpio: ge: fix compilation error

Include linux/slab.h to fix following compilation error.

drivers/gpio/gpio-ge.c: In function ‘gef_gpio_probe’:
drivers/gpio/gpio-ge.c:95:2: error: implicit declaration of function ‘kfree’ [-Werror=implicit-function-declaration]
  kfree(bgc->gc.label);
Signed-off-by: Kamlakant Patel <kamlakant.patel@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-ge.c

@@ -19,6 +19,7 @@
 
 #include <linux/kernel.h>
 #include <linux/io.h>
+#include <linux/slab.h>
 #include <linux/of_device.h>
 #include <linux/of_gpio.h>
 #include <linux/of_address.h>