("@output ".*\n): Close any old yyout before opening a new one.

This fixes Debian bug 156349, reported by Bruce Stephens.

src/scan-skel.l

@@ -60,6 +60,7 @@ int skel_lex (void);
 
   XFREE (outname);
   outname = xstrdup (filename);
+  xfclose (yyout);
   yyout = xfopen (outname, "w");
   lineno = 1;
 }