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 URI_location="LOCAL://">
  <TargetType/>
  <Libraries Enable_Native_Library="true"/>
</BeremizRoot>