bridge: include in6.h in if_bridge.h for struct in6_addr

if_bridge.h uses struct in6_addr ip6, but wasn't including the in6.h
header.  Thomas Backlund originally sent a patch to do this, but this
revealed a redefinition issue: https://lkml.org/lkml/2013/1/13/116



The redefinition issue should have been fixed by the following Linux
commits:
ee262ad8 inet: defines IPPROTO_* needed for module alias generation
cfd280c9 net: sync some IP headers with glibc

and the following glibc commit:
6c82a2f8d7c8e21e39237225c819f182ae438db3 Coordinate IPv6 definitions for Linux and glibc

so actually include the header now.

Reported-by: Colin Guthrie <colin@mageia.org>
Reported-by: Christiaan Welvaart <cjw@daneel.dyndns.org>
Reported-by: Thomas Backlund <tmb@mageia.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>