Glob: Add explicit assignment operator to Message member

 Glob.hxx:58:5: warning: definition of implicit copy assignment operator
  for 'Message' is deprecated because it has a user-declared copy
  constructor [-Wdeprecated]
     Message(const Message& msg) :
     ^

Change-Id: Id6d28d9e5b4ed556556a5add10881f1bcd4525b7