fix get_underlying constexpr-specification mismatch

Summary: Some compilers fail when `friend` declarations have mismatched `constexpr`-specifiers.

Reviewed By: Mizuchi

Differential Revision: D26601032

fbshipit-source-id: a49e327ff9353ddb21eac96776f97d4089c85374

folly/lang/PropagateConst.h

@@ -165,8 +165,8 @@ class propagate_const {
   }
 
  private:
-  friend Pointer& get_underlying<>(propagate_const&);
-  friend Pointer const& get_underlying<>(propagate_const const&);
+  friend constexpr Pointer& get_underlying<>(propagate_const&);
+  friend constexpr Pointer const& get_underlying<>(propagate_const const&);
   template <typename OtherPointer>
   friend class propagate_const;