Add xcompat mod as submodule

.gitmodules

@@ -445,3 +445,6 @@
 [submodule "regional_weather"]
 	path = regional_weather
 	url = https://sys4.fr/gitea/nalc/regional_weather.git
+[submodule "xcompat"]
+	path = xcompat
+	url = https://sys4.fr/gitea/mtcontrib/xcompat.git