Don't forward declare ObjectMaker twice in dynamic

Summary: Makes clang nervous

@override-unit-failures

Test Plan: fbconfig -r folly && fbmake dbg succeeds

Reviewed By: delong.j@fb.com

FB internal diff: D1094209

folly/dynamic.h

@@ -467,7 +467,6 @@ public:
 private:
   friend struct TypeError;
   struct ObjectImpl;
-  struct ObjectMaker;
   template<class T> struct TypeInfo;
   template<class T> struct CompareOp;
   template<class T> struct GetAddrImpl;