ARM: ixp4xx/omixp: always include linux/leds.h

The omixp board code unconditionally defines a gpio-led
device, but for some reason includes the header file
for this only if CONFIG_LEDS_CLASS is enabled, causing
a build error otherwise.

Removing the #ifdef fixes the build error with no downsides
whatsoever.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>