Enable both PulseAudio and Alsa plugins on Linux.

It was previously only one or the other, and all Qt packages for linux
were only including the PulseAudio backend.
Now, both will be included; if PulseAudio is available it will be used
by default, otherwise it will fall back to Alsa.

Change-Id: I393c7728ec9741b1b58681608c184cd1b92126b1
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>