How to rewrite widget function with mixins Magento 2.1.1

requirejs-config.js

var config = {
    config: {
        mixins: {
            'Magento_Swatches/js/swatch-renderer': {
                'path/to/your/mixin': true
            }
        }
    }
};

path/to/your/mixin.js

define([
    'jquery'
], function ($) {
    'use strict';

    return function (widget) {

        $.widget('mage.SwatchRenderer', widget, {
            _Rebuild: function () {
                console.log('Hello from rebuild', arguments);
                return this._super();
            }
        });

        return $.mage.SwatchRenderer;
    }
});

Edit: my answer is not using mixins. To my knowledge, mixins only work for methods rewrites and properties. In your case, it's pure JS being called directly outside of a method.

You can do it via a module.

In Vendor/Module/view/frontend/requirejs-config.js you can add the following:

var config = {
    map: {
        '*': {
            'Magento_Swatches/js/swatch-renderer':'Vendor_Module/js/swatch-renderer'
        }
    }
};

Then you can create the Vendor/Module/view/frontend/web/js/swatch-renderer.js file by copying the original swatch-renderer.js file and modify its content based on what you want to do.