cmake: set advice.detachedHead=false when cloning liburing

cmake: set advice.detachedHead=false when cloning liburing

silence "detached HEAD" warning like:

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

Signed-off-by: Kefu Chai <kchai@redhat.com>