Fix git submodule

Also related to #1

Then you can use to sync the submodule again:
```
git submodule sync
git submodule init
git submodule update
```

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "libserialport"]
 	path = libserialport
-	url = git@github.com:facchinm/libserialport.git
+	url = https://github.com/facchinm/libserialport.git