fix(cdk/schematics): generate code that works in strict CLI projects

- fix tslint error from default CLI settings

Relates to #21981

src/cdk/schematics/ng-generate/drag-drop/files/__path__/__name@dasherize@if-flat__/__name@dasherize__.component.ts.template

@@ -30,7 +30,7 @@ export class <%= classify(name) %>Component {
     'Walk dog'
   ];
 
-  drop(event: CdkDragDrop<string[]>) {
+  drop(event: CdkDragDrop<string[]>): void {
     if (event.previousContainer === event.container) {
       moveItemInArray(event.container.data, event.previousIndex, event.currentIndex);
     } else {