Ignore inconsistent git submodules

Today while building gnome-build-meta I encountered an error with gnome-control-center.

It has recently "added a submodule" by simply updating the .gitmodules file, whilst neglecting to call git submodule add and commit the result.

This can also be a problem when a .bst file explicitly declares a submodule which disappears at track time, the result is an error from the git plugin when trying to find the appropriate commit sha for the said submodule.

Instead, this branch causes the error to be a warning, and the submodule to be ignored.