trivial: synaptics-cxaudio: Add Google Pixel buds
trivial: synaptics-cxaudio: Add Google Pixel buds

I saw a mention that they're actually CX21986 and had some on hand.
They do enumerate:

```
├─Pixel USB-C earbuds:
│     Device ID:           672c087de09848d9e7ee32aa1dea2fbeb8b81e6b
│     Summary:             CX21986 USB audio device
│     Current version:     71.133.20
│     Bootloader Version:  03.01.00.00
│     Vendor:              Google (USB:0x18D1)
│     Install Duration:    3 seconds
│     Flags:               updatable|registered
│     GUIDs:               d76048a5-ca69-5cb8-ac86-d418d70c5f29
│                          98043a29-72c5-549b-ad23-de4e2db20a14
│                          93279fe8-d478-531b-9637-05d026be3c2e
│                          8b71f776-f6d0-549d-9547-42740b24bbbc
```

hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd