src/oping.c: always use "%s"-style format for printf()-style functions

[trofi]

ncuses-6.3 added printf-style function attributes and now makes it easier to catch cases when user input is used in palce of format string when built with CFLAGS=-Werror=format-security:

oping.c:1265:41: error: format not a string literal and no format arguments [-Werror=format-security]
 1265 |                                         hist_symbols_utf8[index]);
      |                                         ^~~~~~~~~~~~~~~~~

Let's wrap all the missing places with "%s" format.

[brd]

This works for me.. @octo Can it be merged?