HID: hid-steelseries fix led class build issue

Fixes 'undefined reference' issue when hid-steelseries is built in,
but led-class is a module.
--
drivers/built-in.o: In function `steelseries_srws1_remove':
hid-steelseries.c:(.text+0x3b97a1): undefined reference to `led_classdev_unregister'
drivers/built-in.o: In function `steelseries_srws1_probe':
hid-steelseries.c:(.text+0x3b9c51): undefined reference to `led_classdev_register'
hid-steelseries.c:(.text+0x3b9ce5): undefined reference to `led_classdev_register'
hid-steelseries.c:(.text+0x3b9d4b): undefined reference to `led_classdev_unregister'
--

Patch allows LED control when led-class is built in, or both hid-steelseries
_and_ led-class are both modules.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Simon Wood <simon@mungewell.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>