plugins: add strstr() to plugin API

Multiple plugins carry their own strstr() implementation because
the in-core version wasn't exported to the plugin API.

Change-Id: Ib57a9b63754c89fb3447ec1d3958963d23145105

apps/plugin.c

@@ -868,6 +868,7 @@ static const struct plugin_api rockbox_api = {
     gesture_vel_process,
     gesture_vel_get,
 #endif
+    strstr,
 };
 
 static int plugin_buffer_handle;