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/interfaces/ecpg/preproc/pgc.l

@@ -84,6 +84,7 @@ static struct _if_value
 %option nodefault
 %option noinput
 %option noyywrap
+%option warn
 
 %option yylineno