Joel Cunningham d785561eae tcpip: guard tcpip_msg enum/struct member with !CORE_LOCKING
enum tcpip_msg_type and struct tcpip_msg have members which are only
used when core locking and/or core locking input are disabled. Remove
these from the union to reduce the size. Remove from enum to prevent
usage when these are options are disabled

A quick sizeof test with MSVC 2013 showed a 4 byte size reduction for
struct tcpip_msg (16 bytes -> 12bytes) for the following configuration:
  LWIP_TCPIP_CORE_LOCKING        1
  LWIP_TCPIP_CORE_LOCKING_INPUT  1
  LWIP_TCPIP_TIMEOUT             0
2018-02-07 16:03:41 -06:00
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