ceph_argparse_flag has no regular 3rd parameter.

With clang warning: 'va_start' has undefined behavior with reference
types, noticing copy-paste mistake on ceph_argparse_flag.

Signed-off-by: Thorsten Behrens <tbehrens@suse.com>
Merge pull request #2068 from thorstenb/wip-janitorial-clang-4

ceph_argparse_flag has no regular 3rd parameter.

Reviewed-by: Loic Dachary <loic@dachary.org>