package/libsamplerate: add optional dependency to alsa-lib

Configure summary without this patch and without alsa-lib built before:

  Extra tools required for testing and examples :
[...]
    Have ALSA : ................... no

and with this patch

  Extra tools required for testing and examples :
[...]
    Have ALSA : ................... yes

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>