fix clang warnings in folly::Gen

Summary: Two clang warnings. It wants user defined consturctor for static initialization, and it wants fwd-declare to agree with full decleration in regards to struct v class.

Test Plan:
`fbconfig --clang --platform=gcc-4.7.1-glibc-2.14.1 folly/test/`

And make sure it doesn't break normal build.

Reviewed By: tulloch@fb.com

FB internal diff: D749857

folly/experimental/Gen-inl.h

@@ -1214,6 +1214,8 @@ class Count : public Operator<Count> {
  */
 class Sum : public Operator<Sum> {
  public:
+  Sum() : Operator<Sum>() {}
+
   template<class Source,
            class Value,
            class StorageType = typename std::decay<Value>::type>

folly/experimental/Gen.h

@@ -250,7 +250,7 @@ template<class Collection>
 class Append;
 
 template<class Value>
-class GeneratorBuilder;
+struct GeneratorBuilder;
 
 template<class Needle>
 class Contains;