SELinux: header generation may hit infinite loop

If a permission name is long enough the selinux class definition generation
tool will go into a infinite loop.  This is because it's macro max() is
fooled into thinking it is dealing with unsigned numbers.  This patch makes
sure the macro always uses signed number so 1 > -1.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>