Move gpiolib-sysfs function declarations into their own header.

These functions are in gpiolib-sysfs.c, and are only required by gpiolib.c,
and so should be in a module header, not gpiolib.h.

This brings gpiolib-sysfs into line with gpiolib-cdev, and is another step
towards removing the sysfs inferface.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>