cmake: Mark UIC-generated .h files to skip AUTOMOC by default.

CMake in 3.10.0 has started warning about autogenerated .h files and
AUTOMOC, and recommends either opting-in to having AUTOMOC run on those
.h files or manually excluding the .h file from AUTOMOC.

Since the ui_*.h files do not create QObject-based subclasses, it seems
to me that matching historical behavior and skipping AUTOMOC is the
proper decision.

Since these ui_*.h files are ultimately setup by the ki18n_wrap_ui CMake
macro, flag the generated .h file as needing to skip AUTOMOC from this
macro.

Differential Revision: https://phabricator.kde.org/D9118