libwps: broaden platforms

Builds fine on Darwin.

pkgs/development/libraries/libwps/default.nix

···
   meta = with stdenv.lib; {
     homepage = http://libwps.sourceforge.net/;
     description = "Microsoft Works document format import filter library";
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     license = licenses.lgpl21;
   };
 }