Bump debhelper compat level from 12 to 13

- debian/rules: Drop --fail-missing argument to dh_missing, which is
  now the default.

Changes-By: lintian-brush
Fixes: lintian: package-uses-old-debhelper-compat-version
See-also: https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html

debian/control

@@ -14,7 +14,7 @@ Build-Depends:
  bsdextrautils <!nocheck> | bsdmainutils (<< 12) <!nocheck>,
  ca-certificates,
  cpio,
- debhelper-compat (= 12),
+ debhelper-compat (= 13),
  dh-exec (>= 0.23~),
  docbook-xml <!nodoc>,
  docbook-xsl <!nodoc>,