LSM: Avoid warnings about potentially unused hook variables

Building with W=1 shows many unused const variable warnings. These can
be silenced, as we're well aware of their being potentially unused:

./include/linux/lsm_hook_defs.h:36:18: error: 'ptrace_access_check_default' defined but not used [-Werror=unused-const-variable=]
   36 | LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
      |                  ^~~~~~~~~~~~~~~~~~~
security/security.c:706:32: note: in definition of macro 'LSM_RET_DEFAULT'
  706 | #define LSM_RET_DEFAULT(NAME) (NAME##_default)
      |                                ^~~~
security/security.c:711:9: note: in expansion of macro 'DECLARE_LSM_RET_DEFAULT_int'
  711 |         DECLARE_LSM_RET_DEFAULT_##RET(DEFAULT, NAME)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/lsm_hook_defs.h:36:1: note: in expansion of macro 'LSM_HOOK'
   36 | LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
      | ^~~~~~~~

Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: KP Singh <kpsingh@chromium.org>
Cc: linux-security-module@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/linux-mm/202110131608.zms53FPR-lkp@intel.com/
Fixes: 98e828a0

 ("security: Refactor declaration of LSM hooks")
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>