extcon: palmas: Fix build break due to devm_gpiod_get_optional API change

With commit b17d1bf1 ("gpio: make flags mandatory for gpiod_get functions")
it becomes necessary to pass the flags argument. And this patch adds the gpio
header file to fix the build breakage when build testing with random
configuration files.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>