[cpp] Remove private specifier from Scratch extern to make it accessible

Was there due to a dodgy copy and paste job

Author: Aidan63

std/cpp/Scratch.hx

@@ -3,7 +3,7 @@ package cpp;
 @:include("hx/thread/Scratch.hpp")
 @:semantics(value)
 @:cpp.ValueType({ namespace : [ "hx", "thread" ], flags : [ StackOnly ] })
-private extern class Scratch {
+extern class Scratch {
 	final view : cpp.marshal.View<cpp.UInt8>;
 
 	static function alloc(bytes:Int):Scratch;