Makefile: add printcounter.cpp too CXXSRC list

The file is required by the optional PRINTCOUNTER feature.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

Marlin/Makefile

@@ -320,7 +320,8 @@ CXXSRC = WMath.cpp WString.cpp Print.cpp Marlin_main.cpp	\
 	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 \
-	dac_mcp4728.cpp vector_3.cpp qr_solve.cpp endstops.cpp stopwatch.cpp utility.cpp
+	dac_mcp4728.cpp vector_3.cpp qr_solve.cpp endstops.cpp stopwatch.cpp utility.cpp \
+	printcounter.cpp
 ifeq ($(LIQUID_TWI2), 0)
 CXXSRC += LiquidCrystal.cpp
 else