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>