plugins: Disk Tidy: wait for dircache

Running Disk Tidy while the dircache is loading is a lot
slower than waiting for it to finish building first.

Typical scenarios for this would be starting Disk Tidy
immediately after booting or after unplugging from USB.

Deleting files during a dircache build also seems to easily
result in memory corruption, as evidenced by visual glitches
appearing in the theme.

Change-Id: I9250d918d916b112ffe2504a9225a0b52bcc3622

apps/plugin.c

@@ -871,6 +871,9 @@ static const struct plugin_api rockbox_api = {
 #endif
     strstr,
     sb_set_title_text,
+#ifdef HAVE_DIRCACHE
+    dircache_wait,
+#endif
 };
 
 static int plugin_buffer_handle;