Include ncurses tinfo library if found.

Include ncurses tinfo library if found.

Thanks to Fabian Groffen for reporting this issue and providing a
couple possible patches.  From Fabian's report:

  For some time now, ncurses can be built in a mode where the low level
  terminfo functionality lives in a separate lib called libtinfo.
  Because some people do, this means Mutt needs to include this
  library in that case to avoid linking errors [...]