bridge: move maybe_deliver_addr() inside #ifdef

The only caller of this new function is inside of an #ifdef checking
for CONFIG_BRIDGE_IGMP_SNOOPING, so we should move the implementation
there too, in order to avoid this harmless warning:

net/bridge/br_forward.c:177:13: error: 'maybe_deliver_addr' defined but not used [-Werror=unused-function]

Fixes: 6db6f0ea ("bridge: multicast to unicast")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>