ima: fix ima_delete_rules() kernel-doc warning

Use correct kernel-doc syntax in the function description to
prevent a kernel-doc warning:

security/integrity/ima/ima_policy.c:1964: warning: expecting prototype for ima_delete_rules() called to cleanup invalid in(). Prototype was for ima_delete_rules() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>