remove LogMessage prototype from plc_main_head.c

it's not needed, because prototype is included in beremiz.h
But it caused problems with TARGET_LOGGING_DISABLE.

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>