gluon-ebtables-limit-arp: declare clock as extern (#2343)

Declare the clock variable as extern in the header file.

Fixes compilation of gluon-ebtables-limit-arp with OpenWrt master:

multiple definition of `clock'; /gluon/openwrt/tmp/ccuynR0G.o:(.bss+0x230): first defined here

Signed-off-by: David Bauer <mail@david-bauer.net>