rtorrent: add ncurses dependency

* otherwise fails with:
  | checking for NcursesW wide-character library... no
  | checking for Ncurses library... no
  | checking for Curses library... no
  | configure: error: requires either NcursesW or Ncurses library
  | Configure failed. The contents of all config.log files follows to aid debugging

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>