greybus: adjust kernel version check for led_sysfs_is_disabled

When building greybus against a 3.18 kernel the following error is generated:
light.c:205:9: error: implicit declaration of function 'led_sysfs_is_disabled'
[-Werror=implicit-function-declaration]

led_sysfs_is_disabled was not added until 3.19 kernel cycle

Verification:
https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/include/linux/leds.h?h=linux-3.18.y


(no function led_sysfs_is_disabled defined here)

Testing Done:
- Successfully built greybus for 3.18 kernel

Signed-off-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>