ima: Fix undefined arch_ima_get_secureboot() and co

Currently arch_ima_get_secureboot() and arch_get_ima_policy() are
defined only when CONFIG_IMA is set, and this makes any code calling
those functions without CONFIG_IMA fail.

Move the declaration and the dummy definition of those functions
outside ifdef-CONFIG_IMA block for fixing the undefined symbols.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
[zohar@linux.ibm.com: removed in-tree/out-of-tree comment in patch description]
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>