Fix compile bug

In file included from tmate-slave.c:18:
tmate.h:101: error: redefinition of typedef ‘ssh_session’
libssh/include/libssh/libssh.h:115: note: previous declaration of ‘ssh_session’ was here
tmate.h:102: error: redefinition of typedef ‘ssh_channel’
libssh/include/libssh/libssh.h:110: note: previous declaration of ‘ssh_channel’ was here
make: *** [tmate-slave.o] Error 1

Signed-off-by: Carl X. Su <bcbcarl@gmail.com>