package/netperf: move SITE so s.b.o

The original site is gone.

Fixes:
http://autobuild.buildroot.net/results/8d4a2ef5a7f50ea21b844d6cf806bb83016ae17b/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>