add systemd networking.service in response to the following lintian warning

E: ifupdown2: systemd-no-service-for-init-rcS-script networking
N:
N:    The rcS init.d script has no systemd equivalent.
N:
N:    Systemd has a SysV init.d script compatibility mode. It provides access
N:    to each SysV init.d script as long as there is no native service file
N:    with the same name (e.g. /lib/systemd/system/rsyslog.service corresponds
N:    to /etc/init.d/rsyslog).
N:
N:    Services in rcS.d are particularly problematic, because they often cause
N:    dependency loops, as they are ordered very early in the boot sequence.
N:
N:    Refer to https://wiki.debian.org/Teams/pkg-systemd/rcSMigration for
N:    details.
N:
N:    Severity: serious, Certainty: certain
N:
N:    Check: systemd, Type: binary

debian/rules

@@ -12,4 +12,5 @@ override_dh_installman:
 	dh_installman
 
 override_dh_installinit:
+	dh_systemd_enable --name=networking
 	dh_installinit --name=networking --no-start