mem-ruby: Avoid const from member due to &Message::operator=(...)

Change-Id: I172f48ce8ee4a3870165309342dadc2ac39ded9a
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28249
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/ruby/slicc_interface/Message.hh

@@ -104,7 +104,7 @@ class Message
     void setVnet(int net) { vnet = net; }
 
   private:
-    const Tick m_time;
+    Tick m_time;
     Tick m_LastEnqueueTime; // my last enqueue time
     Tick m_DelayedTicks; // my delayed cycles
     uint64_t m_msg_counter; // FIXME, should this be a 64-bit value?