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>