libmwaw: broaden platforms

Builds fine on Darwin.

pkgs/development/libraries/libmwaw/default.nix

···
     description = ''Import library for some old mac text documents'';
     license = stdenv.lib.licenses.mpl20 ;
     maintainers = [stdenv.lib.maintainers.raskin];
-    platforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.unix;
   };
 }