windows: add more meta

‘platforms.windows’ includes all windows platforms. Should prefer this
to using assertions.

pkgs/os-specific/windows/mingwrt/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, callPackage, lib, fetchurl }:
+{ stdenv, lib, fetchurl }:
 
 stdenv.mkDerivation rec {
   name = "mingwrt-5.0.2";
@@ -8,7 +8,9 @@ stdenv.mkDerivation rec {
     sha256 = "02pydg1m8y35nxb4k34nlb5c341y2waq76z42mgdzlcf661r91p0";
   };
 
-  meta.platforms = [ lib.systems.inspect.isMinGW ];
+  meta = {
+    platforms = lib.platforms.windows;
+  };
 
   dontStrip = true;
   hardeningDisable = [ "stackprotector" "fortify" ];