Switch to optional priority in Debian package

Switch to using "optional" priority for the Debian package instead of
the deprecated "extra", as recommended by lintian.

Author: spbnick

debian/control

@@ -1,6 +1,6 @@
 Source: digimend-dkms
 Section: kernel
-Priority: extra
+Priority: optional
 Maintainer: Nikolai Kondrashov <spbnick@gmail.com>
 Build-Depends: debhelper (>= 9), dkms
 Standards-Version: 3.9.5