net: l2: ethernet: Fix unused function warning

Building with clang warns:

subsys/net/l2/ethernet/ethernet.c:178:18: error: unused function
'ethernet_check_ipv4_bcast_addr' [-Werror,-Wunused-function]
enum net_verdict ethernet_check_ipv4_bcast_addr(struct net_pkt *pkt,
                 ^

ethernet_check_ipv4_bcast_addr is called by ethernet_ip_recv, which only
exists when CONFIG_NET_IPV4 or CONFIG_NET_IPV6 is defined.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>