package/brltty: add polkit optional dependency

polkit is an optional dependency (enabled by default) since version 5.4:
https://github.com/brltty/brltty/commit/e62b3c925d03239a372d425fb87b2cac65d8ef19



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>