lintian: Use optional priority

Change-Id: Ib397687866b47acf1cbbc6f77bf604b560a2cd2f
(cherry picked from commit 77620b41e6ddea9569ac2be9d35447761974db46)
diff --git a/debian/control b/debian/control
index c17ad00..84b46a1 100644
--- a/debian/control
+++ b/debian/control
@@ -12,7 +12,7 @@
 Package: wayland-protocols
 Section: libdevel
 Architecture: all
-Priority: extra
+Priority: optional
 Depends:
  ${shlibs:Depends},
  ${misc:Depends}