wdiff: Widen meta.platforms to unix

This package builds and works fine on macOS, and probably other
unix-likes.

pkgs/tools/text/wdiff/default.nix

···
     description = "Comparing files on a word by word basis";
     license = stdenv.lib.licenses.gpl3Plus;
     maintainers = [ stdenv.lib.maintainers.eelco ];
-    platforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.unix;
   };
 }