Allow command line compilation when using BEEPER.

Marlin/Makefile

@@ -213,7 +213,7 @@ CXXSRC = WMath.cpp WString.cpp Print.cpp Marlin_main.cpp	\
 	SdFile.cpp SdVolume.cpp motion_control.cpp planner.cpp		\
 	stepper.cpp temperature.cpp cardreader.cpp ConfigurationStore.cpp \
 	watchdog.cpp
-CXXSRC += LiquidCrystal.cpp ultralcd.cpp SPI.cpp Servo.cpp
+CXXSRC += LiquidCrystal.cpp ultralcd.cpp SPI.cpp Servo.cpp Tone.cpp
 
 #Check for Arduino 1.0.0 or higher and use the correct sourcefiles for that version
 ifeq ($(shell [ $(ARDUINO_VERSION) -ge 100 ] && echo true), true)