fix the heap-vector test custom allocator

Summary: It is missing `rebind` which at least some versions of libstdc++ with g++ seem to require.

Reviewed By: dmm-fb

Differential Revision: D56971559

fbshipit-source-id: 33fe5452702c9a5d385e236035ba0e6a02fbfe47

folly/container/test/heap_vector_types_test.cpp

@@ -71,6 +71,11 @@ struct CountingAllocator : std::allocator<T> {
     return std::allocator<T>::allocate(n);
   }
   int nAllocations{0};
+
+  template <typename U>
+  struct rebind {
+    using other = CountingAllocator<U>;
+  };
 };
 
 struct CountCopyCtor {