seccomp: make function __get_seccomp_filter static

The function __get_seccomp_filter is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol '__get_seccomp_filter' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Fixes: 66a733ea ("seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get_filter()")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>