Add membar_sync abi change

It appears membar_sync was not present in libzfs.abi with other
membar_* functions. This commit updates libzfs.abi for membar_sync.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #13969