autogen.sh: git submodule sync

It is not enough to git submodule update on a existing git clone: if the
URL of the repository has been changed, the

   git submodule sync

command must be called to synchronizes submodules' remote URL
configuration setting to the value specified in .gitmodules.

http://tracker.ceph.com/issues/10173 Fixes: #10173

Signed-off-by: Loic Dachary <ldachary@redhat.com>
don't we want &&, so that e.g. the base case where the repo doesn't exist yet still works (in that case presumably sync will fail)?