windows: add more meta

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

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

···
     url = "mirror://sourceforge/mingw/Other/UserContributed/regex/mingw-regex-${version}/mingw-${name}-src.tar.gz";
     sha256 = "0xjxcxgws3bblybw5zsp9a4naz2v5bs1k3mk8dw00ggc0vwbfivi";
   };
+
+  meta = {
+    platforms = stdenv.lib.platforms.windows;
+  };
 }

pkgs/os-specific/windows/mingw-w64/default.nix

···
   dontStrip = true;
   hardeningDisable = [ "stackprotector" "fortify" ];
   patches = [ ./osvi.patch ];
+
+  meta = {
+    platforms = stdenv.lib.platforms.windows;
+  };
 }

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

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

pkgs/os-specific/windows/w32api/default.nix

···
     sha256 = "09rhnl6zikmdyb960im55jck0rdy5z9nlg3akx68ixn7khf3j8wb";
   };
 
-  meta.platforms = [ lib.systems.inspect.isMinGW ];
+  meta = {
+    platforms = lib.platforms.windows;
+  };
 
   dontStrip = true;
 }

pkgs/os-specific/windows/wxMSW-2.8/default.nix

···
 , hostPlatform
 }:
 
-assert hostPlatform.isWindows;
-
 stdenv.mkDerivation {
   name = "wxMSW-2.8.11";
 
···
   ";
 
   passthru = {inherit compat24 compat26 unicode;};
+
+  meta = {
+    platforms = stdenv.lib.platforms.windows;
+  };
 }