Fix sed script to insert Amazon NTP server into ntp.conf once rather

than twice.

Reported by:	Rafal Lukawiecki
MFC after:	1 week

release/tools/ec2.conf

@@ -96,7 +96,7 @@ vm_extra_pre_umount() {
 
 	# Use the NTP service provided by Amazon
 	sed -i '' -e 's/^pool/#pool/' \
-		-e 's/^#server.*/server 169.254.169.123 iburst/' \
+		-e '1,/^#server/s/^#server.*/server 169.254.169.123 iburst/' \
 		${DESTDIR}/etc/ntp.conf
 
 	# The first time the AMI boots, the installed "first boot" scripts