lev_comp: Fix yyparse() declaration in lev_comp.l

Reported by BurnZeZ on GitHub (issue #69).  I don't get this error
myself, nor can I figure out how to raise it, even though the
declaration is clearly wrong, so hopefully this fixes it.

This is meant to go with commit 76ef594e8a (CS456: lev_comp: real
subroutines + more from SporkHack).