add missing rsyslog to apt install line

see #1438

CHANGELOG.md

@@ -7,6 +7,7 @@ In Development
 Setup:
 
 * Update to Roundcube 1.3.8.
+* Add missing rsyslog package to install line since some OS images don't have it installed by default.
 
 Control Panel:
 

setup/system.sh

@@ -126,7 +126,7 @@ echo Installing system packages...
 apt_install python3 python3-dev python3-pip \
 	netcat-openbsd wget curl git sudo coreutils bc \
 	haveged pollinate unzip \
-	unattended-upgrades cron ntp fail2ban
+	unattended-upgrades cron ntp fail2ban rsyslog
 
 # ### Add PHP7 PPA