apparmor: move context.h to cred.h

Now that file contexts have been moved into file, and task context
fns() and data have been split from the context, only the cred context
remains in context.h so rename to cred.h to better reflect what it
deals with.

Signed-off-by: John Johansen <john.johansen@canonical.com>
diff --git a/security/apparmor/policy.c b/security/apparmor/policy.c
index a158af1..a8e096a 100644
--- a/security/apparmor/policy.c
+++ b/security/apparmor/policy.c
@@ -82,7 +82,7 @@
 
 #include "include/apparmor.h"
 #include "include/capability.h"
-#include "include/context.h"
+#include "include/cred.h"
 #include "include/file.h"
 #include "include/ipc.h"
 #include "include/match.h"