Fix broken build due to "missing braces around initializer" error

Summary: Fix broken build due to "missing braces around initializer" error

Reviewed By: nbronson

Differential Revision: D8541080

fbshipit-source-id: f73f973c575b6d3dad999a454d1306f7608122c7

folly/container/test/F14SetTest.cpp

@@ -1022,7 +1022,7 @@ struct RunAllValueSizeTests {
     S<Key, CharArrayHasher, std::equal_to<Key>, std::allocator<Key>> set;
 
     for (int i = 0; i < 100; ++i) {
-      Key key{static_cast<char>(i)};
+      Key key{{static_cast<char>(i)}};
       set.insert(key);
     }
     while (!set.empty()) {