apparmor: Remove the repeated declaration

Function 'aa_labelset_destroy' and 'aa_labelset_init' are declared
twice, so remove the repeated declaration and unnecessary blank line.

Cc: John Johansen <john.johansen@canonical.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>