package/ntp: increase pool server count in default config

To protect agains 1 falsticker NTP server, the client needs to connect
to at least 4 servers.

Source:
 http://support.ntp.org/bin/view/Support/SelectingOffsiteNTPServers
 5.3.3. Upstream Time Server Quantity

Signed-off-by: Gergely Imreh <imrehg@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ntp/ntpd.etc.conf

@@ -1,5 +1,7 @@
 server 0.pool.ntp.org iburst
 server 1.pool.ntp.org iburst
+server 2.pool.ntp.org iburst
+server 3.pool.ntp.org iburst
 
 # Allow only time queries, at a limited rate, sending KoD when in excess.
 # Allow all local queries (IPv4, IPv6)