Add postfix as build dependency.
While creating a package of sympa the configure script searches for the path of postmap and if it is not found, it falls back to standard path /usr/sbin/postmap

As the correct path is required, that postfix integration will not work like it is described here:
https://sympa-community.github.io/manual/install/configure-mail-server-postfix.html

adding postfix as a build dependency ensures that the correct path for postmap is detected and the file /usr/local/libexec/sympa/sympa_newaliases.pl will have the correct path.

PR: 251143
Approved by: dgeo@centrale-marseille.fr (maintainer)