Adds missing configSchema field to plugin-search-backend

This was causing `loadConfigSchema()` to return the wrong
values for this plugin.

Signed-off-by: Mitchell Hentges <mhentges@spotify.com>

.changeset/thin-yaks-hang.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-search-backend': patch
+---
+
+Add missing `configSchema` to package.json

plugins/search-backend/package.json

@@ -72,5 +72,6 @@
   "files": [
     "dist",
     "config.d.ts"
-  ]
+  ],
+  "configSchema": "config.d.ts"
 }