selinux: fix typo in selinux_netlbl_sctp_sk_clone declaration

A missing 'struct' keyword caused a build error when CONFIG_NETLABEL
is disabled:

In file included from security/selinux/hooks.c:99:
security/selinux/include/netlabel.h:135:66: error: unknown type name 'sock'
 static inline void selinux_netlbl_sctp_sk_clone(struct sock *sk, sock *newsk)
                                                                  ^~~~
security/selinux/hooks.c: In function 'selinux_sctp_sk_clone':
security/selinux/hooks.c:5188:2: error: implicit declaration of function 'selinux_netlbl_sctp_sk_clone'; did you mean 'selinux_netlbl_inet_csk_clone'? [-Werror=implicit-function-declaration]

Fixes: db97c9f9d312 ("selinux: Add SCTP support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Moore <paul@paul-moore.com>