Use relative path for Git submodules

laitingsheng · laitingsheng · commit f470fbc1a0e5 · 2023-12-31T11:51:21.000+08:00
diff --git a/.gitmodules b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "src/externals/paho-mqtt-c"]
 	path = src/externals/paho-mqtt-c
-	url = https://github.com/eclipse/paho.mqtt.c.git
+	url = ../paho.mqtt.c.git
