Adds the element contained in the PacketExtension to the child element of the IQ packet. IQ packets, unlike the other packet types, have a unique child element that holds the packet extensions. If an extension is added to an IQ packet that does not have a child element then an IllegalStateException will be thrown.
It is important that this is the first and last time the element contained in PacketExtension is added to another Packet. Otherwise, a runtime error will be thrown when trying to add the PacketExtension's element to the Packet's element. Future modifications to the PacketExtension will be reflected in this Packet.
Note: packet extensions on IQ packets are only for use in specialized situations. In most cases, you should only need to set the child element of the IQ.
@param extension the PacketExtension whose element will be added to this Packet's element.