selinux: fix warning in genheaders

Fix const warning in the genheaders script as a result of
changes to the headers, as noted here:

http://linux.derkeiler.com/Mailing-Lists/Kernel/2010-03/msg03977.html

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: James Morris <jmorris@namei.org>