makefile: add buzzer.cpp (PR#2479)

Marlin/Makefile

 	SdFile.cpp SdVolume.cpp planner.cpp stepper.cpp \
 	temperature.cpp cardreader.cpp configuration_store.cpp \
 	watchdog.cpp SPI.cpp servo.cpp Tone.cpp ultralcd.cpp digipot_mcp4451.cpp \
-	vector_3.cpp qr_solve.cpp
+	vector_3.cpp qr_solve.cpp buzzer.cpp
 ifeq ($(LIQUID_TWI2), 0)
 CXXSRC += LiquidCrystal.cpp
 else