package/iperf3: fix musl build

Ported -D_GNU_SOURCE from https://dev.openwrt.org/changeset/45103



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>