apparmor: mark new functions static

Two new functions were introduced as global functions when they are
only called from inside the file that defines them and should have
been static:

security/apparmor/lsm.c:658:5: error: no previous prototype for 'apparmor_uring_override_creds' [-Werror=missing-prototypes]
security/apparmor/lsm.c:682:5: error: no previous prototype for 'apparmor_uring_sqpoll' [-Werror=missing-prototypes]

Fixes: c4371d90 ("apparmor: add io_uring mediation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John Johansen <john.johansen@canonical.com>