xorg: add meta.platforms

Add it to the generator script (generate-expr-from-tarballs.pl) and then
add the updated generated file (pkgs/servers/x11/xorg/default.nix) after
performing the steps outlined in the generator script.

NOTE: People seem to have updated .../default.nix manually (even though
it has a "THIS IS A GENERATED FILE.  DO NOT EDIT!" header), so I had to
skip some changes to not downgrade several packages.

pkgs/servers/x11/xorg/generate-expr-from-tarballs.pl

@@ -280,6 +280,7 @@ foreach my $pkg (sort (keys %pkgURLs)) {
       sha256 = "$pkgHashes{$pkg}";
     };
     buildInputs = [pkgconfig $inputs];$extraAttrs
+    meta.platforms = stdenv.lib.platforms.unix;
   }) // {inherit $inputs;};
 
 EOF