blktrace: print correct usage

blktrace: print correct usage

Fixup for 498898:

Problem:
When somebody runs blktrace without parameters, it
shows the usage message. The usage message suggests
that version number "x.y.z" is a required parameter,
which is not true.

Solution:
Don't print version number when running
blktrace, blkparce, btt without parameters.

Signed-off-by: Edward Shishkin <edward@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>