xfrm: kconfig: make xfrm depend on inet

when CONFIG_INET is not enabled:
net/xfrm/xfrm_output.c: In function ‘xfrm4_tunnel_encap_add’:
net/xfrm/xfrm_output.c:234:2: error: implicit declaration of function ‘ip_select_ident’ [-Werror=implicit-function-declaration]
ip_select_ident(dev_net(dst->dev), skb, NULL);

XFRM only supports ipv4 and ipv6 so change dependency to INET and place
user-visible options (pfkey sockets, migrate support and the like)
under 'if INET' guard as well.

Fixes: 1de70830 ("xfrm: remove output2 indirection from xfrm_mode")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>