[Patch conntrack-tools] fix error message in configure.in

[Pablo Neira Ayuso]

Eric Leblond wrote:
> The following patch fixes an error message in configure.in. configure
> was asking to install bison instead of flex if flex test failed.

Applied. Thanks Eric.

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The dawn of the fourth age of Linux firewalling is coming; a time of
great struggle and heroic deeds -- J.Kadlecsik got inspired by J.Morris