corkscrew: Fix build with gcc 4.8

gcc 4.8 has a new warning for unused parameters - we need to override
-Werror on that.

Change-Id: Ia2a831a96750ae105440291c636b80382a5702fa
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
More libcorkscrew fixes for GCC 4.8+