ping: fix -Wformat-truncating warning with GCC

Increase buffer size for the string representation of n_time

ICMP timestamp is a 32-bit number. In pr_ntime(), number of minutes
and seconds is always 2 characters wide. Max. number of hours is 4
characters wide. The buffer size should be at least:

4 + 2 + 2 + 1 (':') + 1 (':') + 1 ('\0') = 11

Submitted by: Ján Sučan <sucanjan@gmail.com>
MFC after: 2 weeks
Sponsored by: Google, inc. (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21325