Use relative paths for axc and libomemo submodules

Signed-off-by: Olivier Mehani <shtrom@ssji.net>

.gitmodules

 [submodule "lib/axc"]
 	path = lib/axc
-	url = https://github.com/gkdr/axc
+	url = ../axc
 [submodule "lib/libomemo"]
 	path = lib/libomemo
-	url = https://github.com/gkdr/libomemo
+	url = ../libomemo