buildroot

package/knock: bump version to 0.7.8

Upstream source repo switched to github.com. Since the last release
dating back to 2014 no new tarball was released so we switch to the
latest github commit.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>