use https as submodule url

git:// domains might not be allowed in all environments, while cloning
https through a proxy is fine. -> Make checkout in restrictive
environments possible.

.gitmodules

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