net: offload_ip: Update net_context_accept_cb_t to net_tcp_accept_cb_t

This is required to allow an "IP Offload" driver to build,
once CONFIG_NET_L2_OFFLOAD_IP is enabled.

This fixes the offload_ip.h header following the change made by
commit eb9055c0
("net: tcp: move accept_cb from net_context to net_tcp")

Change-Id: I1a6010c2dbdfc5e74a2ae172aa0167783f4a0cfe
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>