bridge: Fix build of ipv6 multicast code.

Based upon a report from Stephen Rothwell:

--------------------
net/bridge/br_multicast.c: In function 'br_ip6_multicast_alloc_query':
net/bridge/br_multicast.c:469: error: implicit declaration of function 'csum_ipv6_magic'

Introduced by commit 08b202b6726459626c73ecfa08fcdc8c3efc76c2 ("bridge
br_multicast: IPv6 MLD support") from the net tree.

csum_ipv6_magic is declared in net/ip6_checksum.h ...
--------------------

Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -28,6 +28,7 @@
 #include <net/ipv6.h>
 #include <net/mld.h>
 #include <net/addrconf.h>
+#include <net/ip6_checksum.h>
 #endif
 
 #include "br_private.h"