No special changes, just get in sync with recent code.
See here for the changelog:
http://software.es.net/iperf/news.html#iperf-3-9-released
The ipkg sizes changes as follows for mips 24kc :
3.7 : iperf3_3.7-1_mips_24kc.ipk 39675
3.9 : iperf3_3.9-1_mips_24kc.ipk 41586
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>