Temporary fix for alsa-lib breakage with --disable-pcm

The current version of alsa-lib (1.0.22) does not build when
--disable-pcm is used at configure time. I've reported the issue at
https://bugtrack.alsa-project.org/alsa-bug/view.php?id=4913

.

In the mean time, we can prevent the user from not selecting PCM
support by using the following patch.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>