xfrm: fix unused variable warning if CONFIG_NETFILTER=n

After recent change 'x' is only used when CONFIG_NETFILTER is set:

net/ipv4/xfrm4_output.c: In function '__xfrm4_output':
net/ipv4/xfrm4_output.c:19:21: warning: unused variable 'x' [-Wunused-variable]
   19 |  struct xfrm_state *x = skb_dst(skb)->xfrm;

Expand the CONFIG_NETFILTER scope to avoid this.

Fixes: 2ab6096d

 ("xfrm: remove output_finish indirection from xfrm_state_afinfo")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>