The contents of the "ubertooth-2020-12-R1.tar.xz" that is used in the build instructions is out of date with the GitHub repository. Especially in the firmware/bluetooth_rxtx directory. A new "ubertooth-2020-12-R1.tar.xz" needs to be created so people who rebuild the firmware get the correct .c and .h files for the later compiler toolsets.
Ie: wget https://github.com/greatscottgadgets/ubertooth/releases/download/2020-12-R1/ubertooth-2020-12-R