Increase buffer size for the string representation of n_time
This fixes -Wformat-truncation warning when compiled with GCC.
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 <firstname.lastname@example.org>
Sponsored by: Google, inc. (Google Summer of Code 2019)