skmsg: Make __sk_psock_purge_ingress_msg() static

It is only used within skmsg.c so can become static.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210223184934.6054-8-xiyou.wangcong@gmail.com