xfrm: policy: fix IPv6-only espintcp compilation

In case we're compiling espintcp support only for IPv6, we should
still initialize the common code.

Fixes: 26333c37 ("xfrm: add IPv6 support for espintcp")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>