Fix compilation warning

Summary:
Defining hash as a struct and a class was causing some junk to
get printed during fbmake.

Test Plan: `fbconfig -r proxygen; fbmake dbg`, ran unit tests, didn't see output

Reviewed By: njormrod@fb.com

Subscribers: doug, shilin, njormrod

FB internal diff: D1514793

folly/Hash.h

@@ -389,7 +389,7 @@ namespace std {
   // Hash function for pairs. Requires default hash functions for both
   // items in the pair.
   template <typename T1, typename T2>
-  class hash<std::pair<T1, T2> > {
+  struct hash<std::pair<T1, T2> > {
   public:
     size_t operator()(const std::pair<T1, T2>& x) const {
       return folly::hash::hash_combine(x.first, x.second);