mysql service: specify a default package

A config ought to be buildable using just the defaults.

nixos/modules/services/databases/mysql.nix

@@ -52,6 +52,7 @@ in
 
       package = mkOption {
         type = types.package;
+        default = pkgs.mysql;
         example = literalExample "pkgs.mysql";
         description = "
           Which MySQL derivation to use.