tomoyo: fix broken dependency on *.conf.default

tomoyo: fix broken dependency on *.conf.default

If *.conf.default is updated, builtin-policy.h should be rebuilt,
but this does not work when compiled with O= option.

[Without this commit]

  $ touch security/tomoyo/policy/exception_policy.conf.default
  $ make O=/tmp security/tomoyo/
  make[1]: Entering directory '/tmp'
    GEN     Makefile
    CALL    /home/masahiro/ref/linux/scripts/checksyscalls.sh
    DESCEND objtool
  make[1]: Leaving directory '/tmp'

[With this commit]

  $ touch security/tomoyo/policy/exception_policy.conf.default
  $ make O=/tmp security/tomoyo/
  make[1]: Entering directory '/tmp'
    GEN     Makefile
    CALL    /home/masahiro/ref/linux/scripts/checksyscalls.sh
    DESCEND objtool
    POLICY  security/tomoyo/builtin-policy.h
    CC      security/tomoyo/common.o
    AR      security/tomoyo/built-in.a
  make[1]: Leaving directory '/tmp'

$(srctree)/ is essential because $(wildcard ) does not follow VPATH.

Fixes: f02dee2d148b ("tomoyo: Do not generate empty policy files")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>