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/ipc.c b/security/apparmor/ipc.c
index 9013a78..649fad8 100644
--- a/security/apparmor/ipc.c
+++ b/security/apparmor/ipc.c
@@ -100,7 +100,7 @@
 
 	if (!unconfined(tracer_p)) {
 		/* lcred released below */
-		struct cred *lcred = get_task_cred(tracee);
+		const struct cred *lcred = get_task_cred(tracee);
 		struct aa_profile *tracee_p = aa_cred_profile(lcred);
 
 		error = aa_may_ptrace(tracer, tracer_p, tracee_p, mode);