Tmux should look for tinfo in -ltinfo (#15282)

Just `depend_on('ncurses')`, pkgconfig seems to cause the right thing
to happen if the package is built +termlib or ~termlib (tested both
ways on a CentOS 7 system).

Additional details in: https://github.com/spack/spack/issues/15281

Tested by building all of the releases of tmux on a CentOS 7 box.