Fix building F14SetTest on platforms where std::array iterators are not just raw char*'s

Summary: Platforms such as MSVC.

Reviewed By: yfeldblum

Differential Revision: D9569093

fbshipit-source-id: 3d836d5f660b610eed47a6692bc0ce33396fbfa5

folly/container/test/F14SetTest.cpp

@@ -1099,7 +1099,8 @@ namespace {
 struct CharArrayHasher {
   template <std::size_t N>
   std::size_t operator()(std::array<char, N> const& value) const {
-    return folly::Hash{}(StringPiece{value.begin(), value.end()});
+    return folly::Hash{}(
+        StringPiece{value.data(), &value.data()[value.size()]});
   }
 };