libxml2: support all platforms

Mingw/Windows support works out of the box.

pkgs/development/libraries/libxml2/default.nix

@@ -70,7 +70,7 @@ in stdenv.mkDerivation rec {
     homepage = http://xmlsoft.org/;
     description = "An XML parsing library for C";
     license = lib.licenses.mit;
-    platforms = lib.platforms.unix;
+    platforms = lib.platforms.all;
     maintainers = [ lib.maintainers.eelco ];
   };
 }