Fix broken build

Summary: Fix broken build

Reviewed By: ispeters, Orvid

Differential Revision: D26138456

fbshipit-source-id: 8f46b268ecaf923e507db6706af7840095a151f7

folly/functional/test/InvokeTest.cpp

@@ -72,7 +72,7 @@ struct Obj {
 void swap(Obj&, Obj&) noexcept {} // no-op
 } // namespace swappable
 
-struct AltSwappable;
+struct AltSwappable {};
 struct AltSwappableRet {};
 namespace unswappable {
 FOLLY_MAYBE_UNUSED AltSwappableRet swap(AltSwappable&, AltSwappable&);