fix: ubuntu 18.04, service, Exec format error (#8680)

Signed-off-by: fufesou <linlong1266@gmail.com>

res/DEBIAN/postinst

@@ -16,6 +16,12 @@ if [ "$1" = configure ]; then
 		parsedVersion=$(echo "${version//./}")
         mkdir -p /usr/lib/systemd/system/
 		cp /usr/share/rustdesk/files/systemd/rustdesk.service /usr/lib/systemd/system/rustdesk.service
+		# try fix error in Ubuntu 18.04
+		# Failed to reload rustdesk.service: Unit rustdesk.service is not loaded properly: Exec format error.
+		# /usr/lib/systemd/system/rustdesk.service:10: Executable path is not absolute: pkill -f "rustdesk --"
+		if [ -e /usr/bin/pkill ]; then
+			sed -i "s|pkill|/usr/bin/pkill|g" /usr/lib/systemd/system/rustdesk.service
+		fi
 		systemctl daemon-reload
 		systemctl enable rustdesk
 		systemctl start rustdesk