pfixtools: set -Wno-error=unused-result

hardening enables further warnings breaking the build

pkgs/servers/mail/postfix/pfixtools.nix

···
                       --replace /bin/bash ${bash}/bin/bash;
   '';
 
+  NIX_CFLAGS_COMPILE = "-Wno-error=unused-result";
+
   makeFlags = "DESTDIR=$(out) prefix=";
 
   meta = {