Make it explicit that the probe function doesn't modify passed message

src/ypipe_conflate.hpp

@@ -107,7 +107,7 @@ namespace zmq
         //  Applies the function fn to the first elemenent in the pipe
         //  and returns the value returned by the fn.
         //  The pipe mustn't be empty or the function crashes.
-        inline bool probe (bool (*fn)(T &))
+        inline bool probe (bool (*fn)(const T &))
         {
             return dbuffer.probe (fn);
         }