dtoolbase: The default pvector constructor shouldn't be explicit

dtool/src/dtoolbase/pvector.h

@@ -46,7 +46,8 @@ public:
   typedef std::vector<Type, allocator> base_class;
   typedef typename base_class::size_type size_type;
 
-  explicit pvector(TypeHandle type_handle = get_type_handle(pvector<Type>)) : base_class(allocator(type_handle)) { }
+  pvector() : base_class(allocator(get_type_handle(pvector<Type>))) { }
+  explicit pvector(TypeHandle type_handle) : base_class(allocator(type_handle)) { }
   pvector(const pvector<Type> &copy) : base_class(copy) { }
   pvector(pvector<Type> &&from) noexcept : base_class(std::move(from)) {};
   explicit pvector(size_type n, TypeHandle type_handle = get_type_handle(pvector<Type>)) : base_class(n, Type(), allocator(type_handle)) { }