This commit removes two kinds of cmake warning messages:
    1. qmlplugindump failed to load QtGamePad
    2. QtGamePad runtime dependency is unmet

You can check where the qmlplugindump tries to load plugins with shell
environment variable QML_IMPORT_TRACE=1.

In my case, the qmlplugindump tries to load plugins from
$HOME/Qt/5.12.8/gcc_64/qml and there is no directory named QtGamePad.

Instead of that, i have QtGamepad direcory. so i changed module name and
version number in CmakeLists.txt and the warning message disappears.

I don't know exact side-effect of this commit but i think it's worth to
leave this commit here as a MR.