refactor: fix missing abstract directive for date-selection-model

We recently fixed all missing abstract directive decorators in
the components repo. Unfortunately though, the PR was not based
on top of the datepicker that landed while the PR was waiting
for merge. This means that master currently fails, and this commit
fixes the lint failure.

Author: devversion

src/material/datepicker/date-selection-model.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {FactoryProvider, Injectable, Optional, SkipSelf, OnDestroy} from '@angular/core';
+import {FactoryProvider, Injectable, Optional, SkipSelf, OnDestroy, Directive} from '@angular/core';
 import {DateAdapter} from '@angular/material/core';
 import {Observable, Subject} from 'rxjs';
 
@@ -42,6 +42,7 @@ export interface DateSelectionModelChange<S> {
 }
 
 /** A selection model containing a date selection. */
+@Directive()
 export abstract class MatDateSelectionModel<S, D = ExtractDateTypeFromSelection<S>>
     implements OnDestroy {
   private _selectionChanged = new Subject<DateSelectionModelChange<S>>();