package/gensio: add alsa-lib optional dependency

alsa-lib is an optional dependency which is enabled by default since
version 2.5.3 and
https://github.com/cminyard/gensio/commit/dc9bad87d47588dcdd92ce118fcc4f41d10b671c



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>