lexer: add "noinput" option.

This is to prevent the Envoy build warning:

    INFO: From Compiling external/com_github_envoyproxy_sqlparser/src/parser/flex_lexer.cpp:
    flex_lexer.cpp:3577:16: warning: function 'yyinput' is not needed and will not be emitted [-Wunneeded-internal-declaration]
        static int yyinput (yyscan_t yyscanner)
                   ^
    1 warning generated.

Signed-off-by: Ismo Puustinen <ismo.puustinen@intel.com>

Author: ipuustin

src/parser/flex_lexer.l

@@ -40,6 +40,7 @@ static std::stringstream strbuf;
 
 /* other flags */
 %option noyywrap
+%option noinput
 %option nounput
 %option warn
 %option case-insensitive