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".