Fix "QNetworkRequest header not found".

- The QtNetwork include path prefix is used in QtDownload.h for QNetwork* headers.
- For the QNetworkRequest header there is no QtNetwork inlude path prefix in QtDownload.cpp which gives a "header not found" compile error on ArchLinux.
- The patch adds the missing 'QtNetwork' component for 'Qt4' to the CMake build system.

Signed-off-by: Hanspeter Portner <dev@open-music-kontrollers.ch>