libtaskmanager: Fix build when PipeWire is not installed.

The CMake code is careful not to include PipeWire when it is not
installed, however what it *does* include (taskmanagerplugin) still has
a dependency on Screencasting classes, which prior to this are gated
under the PipeWire check.

Since these classes only require Wayland, not PipeWire, I fix the build
error by reducing the CMake check for PipeWire to only add the
PipeWire-specific bits when PipeWire is present, and to build the
screencasting stuff unconditionally.

The ECM logging code is named after PipeWire but doesn't depend on it
either so that is also built unconditionally to avoid build failures
about a missing "logging.h".

BUG:456655