Fix Search component default prop validation

Changed placeholder prop validation to use a regular function instead of
an arrow function to fix undefined errors where 'this' is undefined.

Signed-off-by: Yoshie Muranaka <yoshiemuranaka@gmail.com>
Change-Id: I315542aae3d53a169767fda0407d8379d1b13027
diff --git a/src/components/Global/Search.vue b/src/components/Global/Search.vue
index d305017..b4be463 100644
--- a/src/components/Global/Search.vue
+++ b/src/components/Global/Search.vue
@@ -30,7 +30,7 @@
   props: {
     placeholder: {
       type: String,
-      default: () => {
+      default: function() {
         return this.$t('global.form.search');
       }
     }