netfilter: fix xt_LED build failure

net/netfilter/xt_LED.c:40: error: field netfilter_led_trigger has incomplete type
net/netfilter/xt_LED.c: In function led_timeout_callback:
net/netfilter/xt_LED.c:78: warning: unused variable ledinternal
net/netfilter/xt_LED.c: In function led_tg_check:
net/netfilter/xt_LED.c:102: error: implicit declaration of function led_trigger_register
net/netfilter/xt_LED.c: In function led_tg_destroy:
net/netfilter/xt_LED.c:135: error: implicit declaration of function led_trigger_unregister

Fix by adding a dependency on LED_TRIGGERS.

Reported-by: Sachin Sant <sachinp@in.ibm.com>
Tested-by: Subrata Modak <tosubrata@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>