Remove explicit struct iovec check (HAVE_IOVEC)

Remove explicit struct iovec check (HAVE_IOVEC)

This macro is currently used only in 3 places in syshead.h
- EXTENDED_SOCKET_ERROR_CAPABILITY is linux specific anyway and
  starts with #if defined(HAVE_LINUX_TYPES_H)
- port share and ip_pktinfo macros depends on sendmsg/recvmsg
  that implicitly also require iovec

So in all three cases we can implicitly assume that iovec is present
and do not need to make this explicit check

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210512131511.1309914-2-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22343.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>