Add FM mode display.

NullDisplay.cpp

@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2016,2018 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2016,2018,2020 by Jonathan Naylor G4KLX
  *
  *   This program is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -60,6 +60,10 @@ void CNullDisplay::setQuitInt()
 {
 }
 
+void CNullDisplay::setFMInt()
+{
+}
+
 void CNullDisplay::writeDStarInt(const char* my1, const char* my2, const char* your, const char* type, const char* reflector)
 {
 #if defined(RASPBERRY_PI)