Update copyright in headers (#14092)

Marlin/src/feature/dac/stepper_dac.cpp

  *
  * Part of Marlin
  *
- * Copyright (c) 2016 MarlinFirmware
+ * Copyright (c) 2019 MarlinFirmware
  *
  * Marlin is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by

Marlin/src/pins/pins_MKS_SGEN.h

 /**
  * Marlin 3D Printer Firmware
- * Copyright (C) 2016 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
- * Copyright (C) 2017 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ * Copyright (C) 2019 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
  *
  * Based on Sprinter and grbl.
  * Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm

Marlin/src/pins/pins_PRINTRBOARD_G2.h

 /**
  * Marlin 3D Printer Firmware
- * Copyright (C) 2016, 2019 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ * Copyright (C) 2019 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
  *
  * Based on Sprinter and grbl.
  * Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm

config/examples/Anet/E16/_Statusscreen.h

 /**
  * Marlin 3D Printer Firmware
- * Copyright (C) 2016 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ * Copyright (C) 2019 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
  *
  * Based on Sprinter and grbl.
  * Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm