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).

pkgs/development/libraries/ncurses/default.nix

@@ -46,6 +46,7 @@ stdenv.mkDerivation (rec {
         ln -svf lib''${lib}w.so.5 $out/lib/lib$lib.so.5
       fi
     done;
+    ln -svf . $out/include/ncursesw
   '' else "";
 
   meta = {