Add "%option warn" to all flex input files that lacked it.

This is recommended in the flex manual, and there seems no good reason
not to use it everywhere.

src/backend/bootstrap/bootscanner.l

@@ -55,6 +55,7 @@ static int	yyline = 1;			/* line number for error reporting */
 %option noinput
 %option nounput
 %option noyywrap
+%option warn
 %option prefix="boot_yy"