chore: fix lint failure (#11347)

Fixes a lint failure that was introduced by a PR being merged in close to another one that introduced new lint rules.

Author: crisbeto

src/material-examples/stepper-vertical/stepper-vertical-example.ts

@@ -1,4 +1,4 @@
-import {Component} from '@angular/core';
+import {Component, OnInit} from '@angular/core';
 import {FormBuilder, FormGroup, Validators} from '@angular/forms';
 
 /**
@@ -9,7 +9,7 @@ import {FormBuilder, FormGroup, Validators} from '@angular/forms';
   templateUrl: 'stepper-vertical-example.html',
   styleUrls: ['stepper-vertical-example.css']
 })
-export class StepperVerticalExample {
+export class StepperVerticalExample implements OnInit {
   isLinear = false;
   firstFormGroup: FormGroup;
   secondFormGroup: FormGroup;