crosstool-ng: depend on gperf

Fixes http://autobuild.buildroot.net/results/c9b87123709e1010bae19c6bdfd219661efdbdfb



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>