qt: don't allow building WebKit on some architectures

WebKit has some architecture specific support, and therefore is not
necessarily available for all architectures. Make sure the Qt WebKit
option cannot be selected on those architectures that are not
supported.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>