Removed redudant parenthesis to remove warnings on gen/Base.h

Summary: `-Wredundant-parens` complains about those.

Reviewed By: luciang

Differential Revision: D19347715

fbshipit-source-id: 7460ff1ee675774e8811db95d12ac02ce7e924ea

folly/gen/Base.h

@@ -156,7 +156,7 @@ class ConstMemberFunction {
 template <class Class, class FieldType>
 class Field {
  public:
-  typedef FieldType(Class::*FieldPtr);
+  typedef FieldType Class::*FieldPtr;
 
  private:
   FieldPtr field_;