mark constexpr member function const to be forward compatible.

Summary:
Since C++14, constexpr non-static member function will not be implicitly marked
const. See the blog post below for more details.
https://akrzemi1.wordpress.com/2013/06/20/constexpr-function-is-not-const/

Reviewed By: mzlee

Differential Revision: D6819658

fbshipit-source-id: 4a756191fca3e744bda3c63c2669e24d6b16861e

folly/functional/ApplyTuple.h

@@ -171,7 +171,7 @@ namespace apply_tuple {
 template <class T>
 struct Construct {
   template <class... Args>
-  constexpr T operator()(Args&&... args) {
+  constexpr T operator()(Args&&... args) const {
     return T(std::forward<Args>(args)...);
   }
 };