hooks/xbps-metadata: only add systemd-service trigger if the buildopt is set.

common/hooks/post-install/04-create-xbps-metadata-scripts.sh

@@ -119,6 +119,7 @@ _EOF
 	#
 	# Handle systemd services.
 	#
+	if [ -n "$build_option_systemd" ]; then
 		if [ -n "${systemd_services}" ]; then
 			_add_trigger systemd-service
 			echo "export systemd_services=\"${systemd_services}\"" >> $tmpf
@@ -137,6 +138,7 @@ _EOF
 			_add_trigger systemd-service
 			echo "export systemd_modules=\"${_mods}\"" >> $tmpf
 		fi
+	fi
 	#
 	# Handle GNU Info files.
 	#