title: ng-bootstrap datepicker calender icon is not displaying
body: <p>I am using angular-cli and ng-bootstrap for date picker. after installing datepicker from npm and adding it to main module, datepicker is working, However icon is not displaying.</p>
  
  <p><a href="https://i.stack.imgur.com/j7pip.png" rel="noreferrer">datepicker</a></p>
  
  <pre><code>&lt;div class="form-inline col-sm-6"&gt; 
     &lt;label class="col-sm-3"&gt;From:&lt;/label&gt; 
     &lt;div class="input-group"&gt; 
        &lt;input 
               class="form-control" 
               placeholder="yyyy-mm-dd" name="dp" 
               [(ngModel)]="model" 
                ngbDatepicker 
               #d="ngbDatepicker"&gt; 
              &lt;button 
                   class="input-group-addon" 
                   (click)="d.toggle()" 
                    type="button"&gt; 
                     &lt;img 
                          src="img/calendar-icon.svg" 
                          style="width: 1.2rem; height: 1rem; cursor: pointer;"/&gt; 
             &lt;/button&gt; 
    &lt;/div&gt; 
  &lt;/div&gt;
  </code></pre>