fan-ctrl: mark as broken until SourceForge is back up

SourceForge is down, so we can't download fan-ctrl.c anymore, and it's
not on sources.buildroot.net nor anywhere else on the Internet
apparently. So let's mark this not-so-important package as broken for
now, until SourceForge goes back to life.

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