files: Make mmc_helper depend on gpio

mmc_helper have an hard dependency on gpio_if.h
gpio(4) isn't in the default x86 kernel and none of the x86
sd/mmc drivers uses mmc_helper so just add a dependency on gpio.

Fixes: 85b3794ceef ("files: Make ext_resources non-optional")
(cherry picked from commit f898e419255a7f2e309cb3ccc567ff84ecba2eea)