mmc: cd-gpio: Include header to pickup exported symbol prototypes

Include the linux/mmc/cd-gpio.h header to pickup the prototypes
for the two exported symbols.

This quiets the sparse warnings:

warning: symbol 'mmc_cd_gpio_request' was not declared. Should it be static?
warning: symbol 'mmc_cd_gpio_free' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>