[IRDA]: Compilation for CONFIG_INET=n case

Found this occasionally. 

The CONFIG_INET=n is hardly ever set, but if it is the 
irlan_eth_send_gratuitous_arp() compilation should produce a 
warning about unused variable in_dev.

Too pedantic? :)

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>