Update PlaybackWindow.cpp

[Pascal-0x90]

The compilation has issues on Ubuntu 19.04 with compiling unless these are included into the cpp file.

[lucamarchiori]

Up, without this fix, it is impossible to compile on linux:

[ 20%] Building CXX object CMakeFiles/playcap.dir/AboutWindow.o
[ 40%] Building CXX object CMakeFiles/playcap.dir/InterfaceWindow.o
[ 60%] Building CXX object CMakeFiles/playcap.dir/main.o
[ 80%] Building CXX object CMakeFiles/playcap.dir/PlaybackWindow.o
/home/user/Downloads/PlayCap-master/PlaybackWindow.cpp: In member function ‘void PlaybackWindow::playbackThread()’:
/home/user/Downloads/PlayCap-master/PlaybackWindow.cpp:339:25: error: ‘usleep’ was not declared in this scope
  339 |                         usleep(SLEEP_MILLIS*1000);
      |                         ^~~~~~
make[2]: *** [CMakeFiles/playcap.dir/build.make:118: CMakeFiles/playcap.dir/PlaybackWindow.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/playcap.dir/all] Error 2
make: *** [Makefile:156: all] Error 2