Use relative path in .gitmodules for assimp repository

We're now mirroring https://github.com/assimp/assimp.git

 to
qt/qtquick3d-assimp.git.

All upstream branches are namespaced under "upstream/". Don't push
changes there - they'll be overwritten. If there's need for custom
changes, request a new branch on gerrit for qt/qtquick3d-assimp.git.

The mirror enables us to use a relative repository path in .gitmodules,
which in turn allows init-repository to init and update this submodule.

Fixes: QTBUG-78771
Change-Id: Ibd30d033924eb2a9c56c198ac6b79a69e0ffed7b
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

.gitmodules

 [submodule "src/3rdparty/assimp/src"]
 	path = src/3rdparty/assimp/src
-	url = https://github.com/assimp/assimp.git
+	url = ../qtquick3d-assimp.git