Fix warnings on msvc. Note that the lack of a return shouldn't cause a problem, because we never actually call yypeek in a case where the return is needed, but it's still more future-proof to have it.

src/engine/scan.c

@@ -218,6 +218,7 @@ int yypeek()
             ungetc( ch, incp->file );
         return ch;
     }
+    return EOF;
 }
 
 /*