iperf3: update to version 3.1.4

"This release fixes a few minor bugs, including a
(non-security-impacting) buffer overflow fix ported
from upstream cjson."
<http://software.es.net/iperf/news.html#iperf-3-1-4-released

>

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>