soccr: clean a bit includes list

soccr.h:
After previous patch <linux/types.h> is not needed anymore,
<netinet/tcp.h> is needed as we test for it in feature tests,
i.e., struct tcp_repair_window is declared there.
Also drop forward-declaration of (struct libsoccr_sk) - it's
declared again below.

soccr.c:
Sort headers by name so errors like twice-including errno.h
will not happen again. Also remove assert.h.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>