Fix ipv4/igmp.c compile with gcc-4 and IP_MULTICAST

Modern versions of gcc do not like case statements at the end of a block
statement: you need at least an empty statement.  Using just a "break;"
is preferred for visual style.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

net/ipv4/igmp.c

@@ -970,6 +970,7 @@ int igmp_rcv(struct sk_buff *skb)
 	case IGMP_MTRACE_RESP:
 		break;
 	default:
+		break;
 	}
 
 drop: