Use of "-s" arg causes WSPR to hang in stock Raspbian Stretch

[mschweiner]

Raspbian Stretch no longer ships standard with the NTP package installed, instead relying on timesyncd. This results in the ./wspr command hanging indefinitely at "Waiting for next WSPR transmission window..." when the "-s" self-calibration argument is passed.

This may not be obvious to new or returning users. Recommend adding the following commands into the README installation section to install and verify NTP sync:

sudo apt install ntp ntpstat
ntpstat