net: bridge builtin vs. ipv6 modular

When configs BRIDGE=y and IPV6=m, this build error occurs:

br_multicast.c:(.text+0xa3341): undefined reference to `ipv6_dev_get_saddr'

BRIDGE_IGMP_SNOOPING is boolean; if it were tristate, then adding
	depends on IPV6 || IPV6=n
to BRIDGE_IGMP_SNOOPING would be a good fix.  As it is currently,
making BRIDGE depend on the IPV6 config works.

Reported-by: Patrick Schaaf <netdev@bof.de>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>