drivers/leds/leds-netxbig: make LEDS_NETXBIG depend on LEDS_CLASS

We call led_classdev_register/led_classdev_unregister in
create_netxbig_led/delete_netxbig_led, thus make LEDS_NETXBIG depend on
LEDS_CLASS.

This patch fixes below build error if LEDS_CLASS is not configured.

LD .tmp_vmlinux1
drivers/built-in.o: In function `create_netxbig_led':
drivers/leds/leds-netxbig.c:350: undefined reference to `led_classdev_register'
drivers/leds/leds-netxbig.c:361: undefined reference to `led_classdev_unregister'
drivers/built-in.o: In function `delete_netxbig_led':
drivers/leds/leds-netxbig.c:313: undefined reference to `led_classdev_unregister'

Signed-off-by: Axel Lin <[e-mail]>
Cc: Richard Purdie <[e-mail]>
Acked-by: Simon Guinot <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/leds/Kconfig

···
 config LEDS_NETXBIG
 	tristate "LED support for Big Network series LEDs"
 	depends on MACH_NET2BIG_V2 || MACH_NET5BIG_V2
+	depends on LEDS_CLASS
 	default y
 	help
 	  This option enable support for LEDs found on the LaCie 2Big