iperf3: Add native support

If you want to use iperf3 to measure your board's IP performance, you
may also want to compile if for the build host, because unlike iperf,
iperf3 isn't readily available as a standard Ubuntu package for example.

Add a BBCLASSEXTEND="native" to the recipe so that you just can build
iperf3-native and have bitbake compile it for you, instead of having to
download, compile and install it manually on the build machine.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>