xfrm: avoid compiler warning when ipv6 is disabled

with CONFIG_IPV6=n:
xfrm_output.c:140:12: warning: 'xfrm6_hdr_offset' defined but not used

Fixes: 9acf4d3b ("xfrm: ipv6: add xfrm6_hdr_offset helper")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_output.c

@@ -137,6 +137,7 @@ static int mip6_rthdr_offset(struct sk_buff *skb, u8 **nexthdr, int type)
 }
 #endif
 
+#if IS_ENABLED(CONFIG_IPV6)
 static int xfrm6_hdr_offset(struct xfrm_state *x, struct sk_buff *skb, u8 **prevhdr)
 {
 	switch (x->type->proto) {
@@ -151,6 +152,7 @@ static int xfrm6_hdr_offset(struct xfrm_state *x, struct sk_buff *skb, u8 **prev
 
 	return ip6_find_1stfragopt(skb, prevhdr);
 }
+#endif
 
 /* Add encapsulation header.
  *