Update mapfix

and make a submodule

Author: gaelysam

.gitmodules

@@ -34,3 +34,6 @@
 [submodule "mods/mysql_auth"]
 	path = mods/mysql_auth
 	url = https://github.com/MinetestForFun/mysql_auth.git
+[submodule "mods/mapfix"]
+	path = mods/mapfix
+	url = https://github.com/minetest-mods/mapfix.git