Prevent core savedObjects plugin from being overridden (#58193)

PR #57452 added an empty savedObjects plugin with the same name as the
core plugin. Due to the way we were spreading into our context coupled
with the fact that we don't get NP's whitelisting of plugins on legacy,
we were overriding the core plugin here.

If this happens again, we should perhaps whitelist our plugins here.

x-pack/legacy/plugins/siem/public/app/app.tsx

@@ -106,6 +106,7 @@ const SiemAppComponent: React.FC<SiemAppComponentProps> = ({ core, plugins }) =>
       storage: new Storage(localStorage),
       ...core,
       ...plugins,
+      savedObjects: core.savedObjects,
     }}
   >
     <StartApp {...compose(core)} />