Use the .gitattributes from the modules for the submodule as well.

This should prevent the .gitattributes file ending up in the src
snapshot.

Task-number: QTBUG-41607
Change-Id: I1735331dcbb5f57d30951f942e18bd9c40c803df
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

.gitattributes

+.tag                    export-subst
+.gitignore              export-ignore
+.gitattributes          export-ignore
+.commit-template        export-ignore