Fix compile bug

libssh/include/libssh/libssh.h:115: error: previous declaration of 'ssh_session' was here
tmate.h:83: error: redefinition of typedef 'ssh_channel'
libssh/include/libssh/libssh.h:110: error: previous declaration of 'ssh_channel' was here
make: *** [tmate-ssh-client.o] Error 1