package/alsa-utils: Add option for alsaloop util

Add libsamplerate as optional dependency, see
alsa-utils-1.1.0/alsaloop/Makefile.am

[Peter: move libsamplerate dependency outside alsaloop conditional]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>