netperf: bump to version 2.7.0

Also add hash file.
And work around gcc 5+ defaulting to gnu99, fixes:

http://autobuild.buildroot.net/results/077/077714a0ee67057abcff680b244228a409243bbf/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>