CMake: fail if `Qt5::GuiPrivate` is not found

CMake: fail if `Qt5::GuiPrivate` is not found

`nextcloud` and `nextcloudCore` depend on three Qt5 components which
aren't mentioned in `find_library`: `Xml`, `Network`, and `GuiPrivate`.
The first two are omitted by mistake, apparently, so this commit just
adds them.

`GuiPrivate` is a special case: it doesn't have its own CMake config, so
adding it to "required components" in `find_package` will always fail
the build. Thus, we implement our own check instead.

Signed-off-by: Alexander Batischev <eual.jp@gmail.com>