gpio: mockup: add sysfs dependency

Building the gpio-mockup driver without SYSFS results in a harmless Kconfig
warning:

warning: (GPIO_MOCKUP) selects GPIO_SYSFS which has unmet direct dependencies (GPIOLIB && SYSFS)

We can easily avoid that warning by adding a dependency on SYSFS.

Fixes: 0f98dd1b ("gpio/mockup: add virtual gpio device")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>