Switch deprecated extra priority to optional

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

debian/control

 Source: psdk
 Section: devel
-Priority: extra
+Priority: optional
 Maintainer: Apertis package maintainers <packagers@lists.apertis.org>
 Build-Depends:
  debhelper (>= 10),