net/sched: avoid unused-label warning

The label is only used from inside the #ifdef and should be
hidden the same way, to avoid this warning:

net/sched/act_tunnel_key.c: In function 'tunnel_key_init':
net/sched/act_tunnel_key.c:389:1: error: label 'release_tun_meta' defined but not used [-Werror=unused-label]
 release_tun_meta:

Fixes: 41411e2f

 ("net/sched: act_tunnel_key: Add dst_cache support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>