netperf: Update build file to v2.6.0, pass --enable-demo to configure.

This makes this version of netperf work with the netperf-wrapper testing tool
used for testing for bufferbloat.

Patch by: Toke Høiland-Jørgensen <toke@toke.dk>

git-svn-id: svn://svn.openwrt.org/openwrt@36341 3c298f89-4303-0410-b956-a3cf2f4a3e73