Add ncursesw compatibility link also for includes

This is needed for the ncurses cabal package to compile.  It uses
include/ncursesw/curses.h, without trying include/curses.h first.  The
files are provided through include/ncursesw on a clean Debian too, in
the unicode case (when the libncursesw5-dev is installed).
Merge pull request #555 from errge/master

Add ncursesw compatibility link also for includes