Add all CPP files to Makefile automatically

Fixes build errors occurring when AUTO_BED_LEVELING_UBL is enabled.

Marlin/Makefile

@@ -304,13 +304,8 @@ ifeq ($(HARDWARE_VARIANT), Teensy)
 SRC = wiring.c
 VPATH += $(ARDUINO_INSTALL_DIR)/hardware/teensy/cores/teensy
 endif
-CXXSRC = WMath.cpp WString.cpp Print.cpp Marlin_main.cpp \
-	MarlinSerial.cpp Sd2Card.cpp SdBaseFile.cpp SdFatUtil.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 least_squares_fit.cpp endstops.cpp stopwatch.cpp utility.cpp \
-	printcounter.cpp nozzle.cpp serial.cpp gcode.cpp Max7219_Debug_LEDs.cpp
+CXXSRC = WMath.cpp WString.cpp Print.cpp SPI.cpp Tone.cpp
+CXXSRC += $(wildcard *.cpp)
 ifeq ($(NEOPIXEL), 1)
 CXXSRC += Adafruit_NeoPixel.cpp
 endif