tcping: fix upstream location

The upstream site is no longer available from http://www.linuxco.de,
but only from http://linuxco.de.

Fixes:

  http://autobuild.buildroot.net/results/03caf8403c31e2424c338de4df636dd900408b14/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>