sstuff: Convert a static const to static constexpr

pdns/sstuff.hh

@@ -416,7 +416,7 @@ public:
   }
 
 private:
-  static const size_t s_buflen{4096};
+  static constexpr size_t s_buflen{4096};
   std::string d_buffer;
   int d_socket;
 };