qt: fix alsa-lib dependency

If BR2_PACKAGE_QT_AUDIO_BACKEND=y, QtMultimedia requires the audio backend
to be already in staging in order to compile.
The backend is system-dependent, but since buildroot builds Linux systems
only, it is safe to assume that the it will always be ALSA.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>