README_BUILD.md: Document WITH_IIOD_USB_DMABUF

Add an entry in the CMake options table for the WITH_IIOD_USB_DMABUF
toggle.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

README_BUILD.md

@@ -108,6 +108,7 @@ Cmake Options       | Default | Description                                    |
 `WITH_IIOD_NETWORK` |  ON | Add network (TCP/IP) support                         |
 `WITH_IIOD_SERIAL`  |  ON | Add serial (UART) support                            |
 `WITH_IIOD_USBD`    |  ON | Add support for USB through FunctionFS within IIOD   |
+`WITH_IIOD_USB_DMABUF` | OFF | Enable DMABUF support on the USB stack            |
 `WITH_IIOD_V0_COMPAT` |  ON | Add support for Libiio v0.x protocol and clients |
 `WITH_AIO`          |  ON | Build IIOD with async. I/O support                   |
 `WITH_SYSTEMD`      | OFF | Enable installation of systemd service file for iiod |