Make every struct have a header header and not inherit the header

pdu/const.go

@@ -156,4 +156,4 @@ var COMMAND_STATUS = struct {
 	ESME_RINVOPTPARAMVAL:  0x000000C4, // Invalid Optional Parameter Value
 	ESME_RDELIVERYFAILURE: 0x000000FE, // Delivery Failure (used for
 	ESME_RUNKNOWNERR:      0x000000FF, // Unknown Error
-}
+}