bridge: Fix build of ipv6 multicast code.

Based upon a report from Stephen Rothwell:

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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 08b202b6 ("bridge
br_multicast: IPv6 MLD support") from the net tree.

csum_ipv6_magic is declared in net/ip6_checksum.h ...
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Signed-off-by: David S. Miller <davem@davemloft.net>