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>