nixos/postfix: Set type for extraConfig to "lines"

Regression introduced by 3891d3e6541fe588ee2430e7b1bdb8d87d787a53.

Merging multiple options with type "str" won't work and give an
evaluation error. For extra configuration lines in the Postfix config it
really should be "lines", especially because even the description
mentions "extra lines".

Signed-off-by: aszlig <[e-mail]>

nixos/modules/services/mail/postfix.nix

···
       };
 
       extraConfig = mkOption {
-        type = types.str;
+        type = types.lines;
         default = "";
         description = "
           Extra lines to be added verbatim to the main.cf configuration file.