Enable qubes-firewall also in "NetVM"

In some cases it may make sense to enfoce outgoing firewall also on
sys-net. If the service is disabled, firewall settings will be
(silently) ignored, so better be on the safe side and enable.

QubesOS/qubes-issues#3290

vm-systemd/qubes-sysinit.sh

@@ -5,7 +5,7 @@
 . /usr/lib/qubes/init/functions
 
 # List of services enabled by default (in case of absence of qubesdb entry)
-DEFAULT_ENABLED_NETVM="network-manager qubes-network qubes-update-check qubes-updates-proxy meminfo-writer"
+DEFAULT_ENABLED_NETVM="network-manager qubes-network qubes-update-check qubes-updates-proxy meminfo-writer qubes-firewall"
 DEFAULT_ENABLED_PROXYVM="qubes-network qubes-firewall qubes-update-check meminfo-writer"
 DEFAULT_ENABLED_APPVM="cups qubes-update-check meminfo-writer"
 DEFAULT_ENABLED_TEMPLATEVM="$DEFAULT_ENABLED_APPVM updates-proxy-setup"