ipv4: Fix build with multicast disabled.

net/ipv4/igmp.c: In function 'ip_mc_inc_group':
net/ipv4/igmp.c:1228: error: implicit declaration of function 'for_each_pmc_rtnl'
net/ipv4/igmp.c:1228: error: expected ';' before '{' token
net/ipv4/igmp.c: In function 'ip_mc_unmap':
net/ipv4/igmp.c:1333: error: expected ';' before 'igmp_group_dropped'
 ...

Move for_each_pmc_rcu and for_each_pmc_rtnl macro definitions
outside of multicast ifdef protection.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>