FrSky XSR-M/XSR-E mount for 30.5 mm stack

The [FrSky XSR-M/XSR-E](https://www.banggood.com/Original-FrskyEACHINE-15x15mm-XSR-E-D16-2_4G-16CH-ACCST-Dual-Receiver-Compatible-SUBS-CPPM-Output-p-1148143.html?p=AI10081335579201503X) mount is a *reaaaallly* tiny receiver (20x20 mm in size with 15x15 mounting), having 16 channels and telemetry. Since I couldn't find a suitable mount/adapter for it (in combination with my frame), here's this little mount which:

- can be printed without supports
- allows mounting the receiver in a standard 3