perf stat: Suppress printing std-dev when its 0

perf stat: Suppress printing std-dev when its 0

For pretty output only (preserve column for cvs output), dont print
std-deviation when its 0.00.  Do this based upon value, instead of
checking for --no-aggr, since the stats could conceivably be computed
over the runs on each CPU, and theres no reason to preclude that.

Link: http://lkml.kernel.org/r/1315437244-3788-4-git-send-email-jim.cromie@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>