videothumbnailer.cpp: add <cstring> include for strerror()

[trofi]

Upcoming gcc-12 cleaned implicit headers includes further and exposed missing include as this build failure:

libffmpegthumbnailer/videothumbnailer.cpp: In member function 'void ffmpegthumbnailer::VideoThumbnailer::writeImage(...)':
libffmpegthumbnailer/videothumbnailer.cpp:274:109: error: 'strerror' was not declared in this scope; did you mean 'stderr'?
  274 |             TraceMessage(ThumbnailerLogLevelError, std::string("Failed to stat file ") + videoFile + " (" + strerror(errno) + ")");