ipv6: remove duplicated declaration of ip6_fragment()

It is declared in:
include/net/ip6_route.h:187:int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *));

and net/ip6_route.h is already included.

Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -56,8 +56,6 @@
 #include <net/checksum.h>
 #include <linux/mroute6.h>
 
-int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *));
-
 int __ip6_local_out(struct sk_buff *skb)
 {
 	int len;