if_ovpn: fix unused functions with NOINET / NOINET6

ovpn_find_peer_by_ip() is not used if INET is not defined. Do not
define the function in that case. Same for ovpn_find_peer_by_ip6().

Fix these warnings:

/usr/src/sys/net/if_ovpn.c:1580:1: warning: unused function 'ovpn_find_peer_by_ip' [-Wunused-function]
ovpn_find_peer_by_ip(struct ovpn_softc *sc, const struct in_addr addr)
^
/usr/src/sys/net/if_ovpn.c:1599:1: warning: unused function 'ovpn_find_peer_by_ip6' [-Wunused-function]
ovpn_find_peer_by_ip6(struct ovpn_softc *sc, const struct in6_addr *addr)
^

Reported by: mjg
Sponsored by: Rubicon Communications, LLC ("Netgate")