TOMOYO: Fix build error with CONFIG_SECURITY_TOMOYO_OMIT_USERSPACE_LOADER=y .

I forgot to add #ifndef in commit 0e4ae0e0 "TOMOYO: Make several options
configurable.", resulting

  security/built-in.o: In function `tomoyo_bprm_set_creds':
  tomoyo.c:(.text+0x4698e): undefined reference to `tomoyo_load_policy'

error.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>