Added calls to sim_register_clock_unit for simulators with clock devices

VAX/vax780_stddev.c

@@ -672,6 +672,7 @@ return;
 
 t_stat clk_reset (DEVICE *dptr)
 {
+sim_register_clock_unit (&clk_unit);                    /* declare clock unit */
 tmr_poll = sim_rtcn_init (clk_unit.wait, TMR_CLK);      /* init 100Hz timer */
 sim_activate (&clk_unit, tmr_poll);                     /* activate 100Hz unit */
 tmxr_poll = tmr_poll * TMXR_MULT;                       /* set mux poll */