kernel-binary.spec: Also exclude the kernel signing key from devel package.
    
    There is a check in OBS that fails when it is included. Also the key is
    not reproducible.
    
    Fixes: bb988d4625a3 ("kernel-binary: Do not include sourcedir in certificate path.")
    Signed-off-by: Michal Suchanek <msuchanek@suse.de>