gnuchess: add missing dependencies on flex and host-flex

And also add a missing newline and licensing informations while we're
at it.

Fixes:
http://autobuild.buildroot.org/results/2851f23990c9f6642b1e706a94b1afc15a24b6f7/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>