AppArmor: fix build warnings for non-const use of get_task_cred

Fix build warnings for non-const use of get_task_cred.

Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/security/apparmor/domain.c b/security/apparmor/domain.c
index 08bbe63..c825c6e 100644
--- a/security/apparmor/domain.c
+++ b/security/apparmor/domain.c
@@ -62,7 +62,7 @@
 				     struct aa_profile *to_profile)
 {
 	struct task_struct *tracer;
-	struct cred *cred = NULL;
+	const struct cred *cred = NULL;
 	struct aa_profile *tracerp = NULL;
 	int error = 0;