net/netaddress: make copy member function const

As the function does not alter the internal state of the
object allow for using it in r/o contexts.

Issue #4813

repos/os/include/net/netaddress.h

@@ -52,7 +52,7 @@ struct Net::Network_address
 	 ** Helper methods  **
 	 *********************/
 
-	void copy(void *dst) { Genode::memcpy(dst, &addr[0], LEN); }
+	void copy(void *dst) const { Genode::memcpy(dst, &addr[0], LEN); }
 
 	void print(Genode::Output &output) const
 	{