Switch from priority extra (deprecated) to optional

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

debian/control

 Source: traprain
 Section: libs
-Priority: extra
+Priority: optional
 Maintainer: Apertis maintainers <maintainers@lists.apertis.org>
 Build-Depends:
  autoconf-archive,