tomoyo: Use if_changed when generating builtin-policy.h

Combine the generation of builtin-policy.h into a single command and use
if_changed, so that the file is regenerated each time the command
changes. The next patch will make use of this.

Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Michal Marek <mmarek@suse.cz>