constify conversion operator

src/node/mac48-address.cc

@@ -95,7 +95,7 @@ Mac48Address::IsMatchingType (const Address &address)
 {
   return address.CheckCompatible (GetType (), 6);
 }
-Mac48Address::operator Address ()
+Mac48Address::operator Address () const
 {
   return ConvertTo ();
 }

src/node/mac48-address.h

@@ -61,7 +61,7 @@ public:
    *
    * Convert an instance of this class to a polymorphic Address instance.
    */
-  operator Address ();
+  operator Address () const;
   /**
    * \param address a polymorphic address
    * \returns a new Mac48Address from the polymorphic address