Add clean rule to art/Makefile

Makefile

@@ -51,4 +51,5 @@ endif
 
 clean:
 	rm -f *.h *.o *.elf $(DISTDIR)/*.gdb $(DISTDIR)/$(EXE) $(OBJS)
+	$(MAKE) -C art/ clean
 

art/Makefile

@@ -4,3 +4,6 @@ all:
 	rm -f sprites.c
 	for FILE in $(SPRITES); do exec ./sprite.py $$FILE >> sprites.c; done
 
+clean:
+	rm -f sprites.c sprites.h
+