Fix 'struct omx_itch186_message' name

Rename 'struct omx_itch186_msg_message' to 'struct omx_itch186_message'.
The latter name is already used elsewhere.

Note that this change most likely breaks all applications depending on
Libtrading for Nordic Equity TotalView-ITCH version 1.86.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>