fix(material-experimental/mdc-slider): fixup

* add license to slider-adapter.ts

Author: wagnermaciel

src/material-experimental/mdc-slider/slider-adapter.ts

@@ -1,3 +1,11 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
 import {SpecificEventListener, EventType} from '@material/base';
 import {MDCSliderAdapter, Thumb, TickMark} from '@material/slider';