CRED: fix build error due to 'tgcred' undeclared

This patch adds CONFIG_KEYS guard for tgcred to fix below build error
if CONFIG_KEYS is not configured.

  CC      kernel/cred.o
kernel/cred.c: In function 'prepare_kernel_cred':
kernel/cred.c:657: error: 'tgcred' undeclared (first use in this function)
kernel/cred.c:657: error: (Each undeclared identifier is reported only once
kernel/cred.c:657: error: for each function it appears in.)
make[1]: *** [kernel/cred.o] Error 1
make: *** [kernel] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

kernel/cred.c

@@ -656,11 +656,13 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon)
 	if (!new)
 		return NULL;
 
+#ifdef CONFIG_KEYS
 	tgcred = kmalloc(sizeof(*tgcred), GFP_KERNEL);
 	if (!tgcred) {
 		kmem_cache_free(cred_jar, new);
 		return NULL;
 	}
+#endif
 
 	kdebug("prepare_kernel_cred() alloc %p", new);