qmldir is installed into source instead of build tree when shadow building

When trying to test SoundEffect element provided by multimedia qml plugin I found that it didn't work since the qmldir file was being copied to the wrong location.
Can use piano.qml to test (in declarative-ui-examples/piano)
I manually copied the qmldir file to the build/import/Qt/multimedia directory and it worked.

NOTE:
If the multimedia library detects pulseaudio files it will build the pulseaudio backend and you will need to make sure pulseaudio daemon is running as well.
eg. in console run: pulseaudio
If pulseaudio files are not present it should use qmedia(gstreamer) on desktop linux (dont need to do anything extra).