Don't ignore the return value from rm -rf

The `-f` option causes `rm` to silently ignore files that don't exist,
so `rm -rf` will only fail if `rm` has trouble deleting a file that
exists.  We want `make` to fail in that situation.

Makefile

@@ -91,7 +91,7 @@ uninstall:
 		$(DESTDIR)$(PREFIX)/bin/keyd-usb-gadget.sh \
 		$(DESTDIR)$(PREFIX)/lib/systemd/system/keyd.service
 clean:
-	-rm -rf bin keyd.service src/vkbd/usb-gadget.service
+	rm -rf bin keyd.service src/vkbd/usb-gadget.service
 test:
 	@cd t; \
 	for f in *.sh; do \