pbio/test: Fix clean makefile.

Otherwise clean always led to errors such as

../drv/bluetooth/bluetooth_btstack.c:25:10: fatal error: genhdr/pybricks_service.h: No such file or directory
   25 | #include "genhdr/pybricks_service.h"

Author: laurensvalk

lib/pbio/test/Makefile

@@ -167,7 +167,9 @@ $(BUILD_PREFIX)/%.d: %.c
 	$(Q)mkdir -p $(dir $@)
 	$(Q)$(CC) $(CFLAGS) -MM -MT $(patsubst %.d,%.o,$@) $< > $@
 
+ifneq ($(MAKECMDGOALS),clean)
 -include $(DEP)
+endif
 
 $(BUILD_PREFIX)/%.o: %.c $(BUILD_PREFIX)/%.d Makefile
 	$(Q)mkdir -p $(dir $@)