portaudio: fix dependency on alsa-lib

BR2_PACKAGE_PORTAUDIO_ALSA is misspelled in one place, resulting in
alsa-lib not being listed in the dependencies.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>