apparmor: declare stack_msg as static

stack_msg in upstream code is only used in securit/apparmor/domain.c
so declare it as static.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311092251.TwKSNZ0u-lkp@intel.com/Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/domain.c

@@ -1311,7 +1311,7 @@ static int change_profile_perms_wrapper(const char *op, const char *name,
 	return error;
 }
 
-const char *stack_msg = "change_profile unprivileged unconfined converted to stacking";
+static const char *stack_msg = "change_profile unprivileged unconfined converted to stacking";
 
 /**
  * aa_change_profile - perform a one-way profile transition