Add missing thread include to ctimepoint (#6)

This resolves the following error compiling on Ubuntu 22.04:

ctimepoint.cpp: In static member function ‘static void CTimePoint::TaskSleepFor(uint)’:
ctimepoint.cpp:52:27: error: ‘sleep_for’ is not a member of ‘std::this_thread’
   52 |         std::this_thread::sleep_for(timespan);
      |                           ^~~~~~~~~

$ gcc -v
<<snip>>
gcc version 11.3.0 (Ubuntu 11.3.0-1ubuntu1~22.04

Co-authored-by: Mark Landis (N6AZX) <n6azx@fastmail.com>

src/ctimepoint.cpp

@@ -25,6 +25,8 @@
 #include "main.h"
 #include "ctimepoint.h"
 
+#include <thread>
+
 ////////////////////////////////////////////////////////////////////////////////////////
 // constructor