cmake: add warning when baloo will be disabled

Summary:
KFileMetaDataWidget is useless without nepomuk (which no one ships
anymore). Let's add at least a warning until we will get rid of
kdelibs4support (see T8720).

Test Plan: Uninstall baloo-widgets, build dolphin and check the cmake output.

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D13005