Copter: emit system time during compassmot on SITL

the autotest framework heavily relies on system_time being emitted -
because we're not running the main loop during compassmot it gets stuck
if we don't do this

ArduCopter/compassmot.cpp

@@ -228,6 +228,10 @@ MAV_RESULT Copter::mavlink_compassmot(const GCS_MAVLINK &gcs_chan)
 #if HAL_WITH_ESC_TELEM
             // send ESC telemetry to monitor ESC and motor temperatures
             AP::esc_telem().send_esc_telemetry_mavlink(gcs_chan.get_chan());
+#endif
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+            // a lot of autotest timeouts are based on receiving system time
+            gcs_chan.send_system_time();
 #endif
         }
     }