ASoC: omap: Make RX51 depend on GPIOLIB due to jack usage

Since the GPIO jacks are only supported if gpiolib is built and fail to
compile otherwise add a build depedency.  This is unlikely to have any
practical impact outside of coverage testing.

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>