Build testptp on Debian

Applies to Debian-based distros too.

1. Install build-essential and kernel source

   sudo apt install linux-source build-essential

2. Decompress kernel source

   cd /usr/src ;
   sudo tar -xf linux-source-5.10.tar.xz

3. Build!

   cd /usr/src/linux-source-5.10/tools/testing/selftests/ptp ;
   gcc -static testptp.c -o testptp

4. Optional - install

   mv testptp /usr/bin/.

5. Test!

   $ sudo testptp /dev/ptp0 -c
   capabilities:
     62499999 maximum frequency adjustment (ppb)
     0 programmable alarms
     2 external time stamp channels
     2 programmable periodic signals
     1 pulse per second
     4 programmable pins
     0 cross timestamping
     0 adjust_phase