Avatar: recover removed css-property (#17372)

This reverts commit 10592d12ea981912165542920160669fd8874bd9.

packages/theme-chalk/src/avatar.scss

@@ -16,6 +16,7 @@
   >img {
     display: block;
     height: 100%;
+    vertical-align: middle;
   }
 
   @include m(circle) {