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>
The documentation suggests it is a noop if .git/config does not exist yet
<pre>
       sync
           Synchronizes submodules' remote URL configuration setting to the value specified in
           .gitmodules. It will only affect those submodules which already have a URL entry in
           .git/config (that is the case when they are initialized or freshly added).
</pre>
and it is verified with
<pre>
loic@fold:/tmp$ git clone https://github.com/ceph/ceph.git
Cloning into 'ceph'...
remote: Counting objects: 267197, done.
remote: Compressing objects: 100% (52759/52759), done.
remote: Total 267197 (delta 212449), reused 267076 (delta 212348)
Receiving objects: 100% (267197/267197), 92.92 MiB | 907.00 KiB/s, done.
Resolving deltas: 100% (212449/212449), done.
Checking connectivity... done.
loic@fold:/tmp$ cd ceph
loic@fold:/tmp/ceph$ git submodule sync
loic@fold:/tmp/ceph$ echo $?
0
</pre>