openssh/auth2.c: Add a missing include (canohost.h).

openssh/auth2.c: Add a missing include (canohost.h).

canohost.h has the prototypes for get_canonical_hostname() and
get_remote_ipaddr(), which are used in auth2.c and which both return
const char * (64 bits on x86_64). Without the prototypes, int is
assumed (32 bits).

This caused sshd(8) to crash on x86_64 (i386 isn't affected due to the
char pointer and int having the same width there).

Dragonfly-bug:  <http://bugs.dragonflybsd.org/issue1649>
Pointed-out-by: dillon