[SNMP4J] Resolving a NotsSerializableException on UsmUserTable.UsmUserKey

[Mark Ellison]

Hi,

I may have found a small bug while working with the UsmUserTable class.  
I received a NotSerializableException due identifying the static inner 
class UsmUserKey.

Adding Serializable as an interface as follows resolved the exception:
    public static class UsmUserKey implements Comparable, Serializable {
       ...
    }

Regards,

Mark