build: allow packpack's build for Debian 11 (bullseye)

Make build dependency on dh-systemd optional, as this package is
deprecated and removed from Debian 11. It part of debhelper (>=
9.20160709) now. No new dependencies, as tarantool already depends on
debhelper (> 9).

debian/control

@@ -4,7 +4,7 @@ Maintainer: Roman Tsisyk <roman@tarantool.org>
 Uploaders: Dmitry E. Oboukhov <unera@debian.org>
 Build-Depends: cdbs (>= 0.4.100), debhelper (>= 9), dpkg-dev (>= 1.16.1~),
 # Enable systemd for Debian Jessie+ and Ubuntu Wily+
- dh-systemd (>= 1.22) | sysvinit (<< 2.88dsf-59) | upstart (<< 1.13),
+ debhelper (>= 9.20160709) | dh-systemd (>= 1.22) | sysvinit (<< 2.88dsf-59) | upstart (<< 1.13),
  cmake,
  libreadline-dev,
  libncurses5-dev,