leds: Only descend into leds directory when CONFIG_NEW_LEDS is set

When CONFIG_NEW_LEDS is not set make will still descend into the leds
directory but nothing will be built. This produces unneeded build
artifacts and messages in addition to slowing the build. Fix this here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>