PUBLIC target_compile_options for VirtualKeyboard module

when adding VirtualKeyboard module to the QML, the VirtualKeyboard CMake files perform a PUBLIC target_compile_options call, which adds -Werror, that propagates to all our CPP files linking Qul. Moreover, it is also public linking Qul::Core which also pushes the -Werror flag to the rest of project using Qul.

If you look at other modules, VirtualKeboard is the only module having this behavior (Controls, Shapes and other modules do not perform any PUBLIC compile option forcing).

 Like the other modules, the compile options required by VirtualKeyboard should be PRIVATE and not PUBLIC, otherwise it could break all the customer compile options...