fix($theme-default): fix height of input in SearchBox for IE11 (#1093)

Author: deraw

packages/@vuepress/plugin-search/SearchBox.vue

@@ -160,6 +160,7 @@ export default {
   input
     cursor text
     width 10rem
+    height: 2rem
     color lighten($textColor, 25%)
     display inline-block
     border 1px solid darken($borderColor, 10%)