Use a relative submodule url for src/3rdparty

Now that the CI configuration has been updated to support them,
use a relative submodule URL to make sure that we use the same
mirror for our submodule that was used for qtwebengine.

Change-Id: I92215bd3caef2bb33820185572f5bb088718f61f
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>

.gitmodules

 [submodule "src/3rdparty"]
     path = src/3rdparty
-    url = git://gitorious.org/qt/qtwebengine-chromium.git
+    url = ../qtwebengine-chromium.git
     ignore = all