mtr: fix build against upcoming ncurses-6.3
mtr: fix build against upcoming ncurses-6.3

On ncurses-6.3 with extra printf() annotations gcc now detects
use of user input in place of format strings:

    ui/curses.c:765:42:
      error: format not a string literal and no format arguments [-Werror=format-security]
      765 |         mvprintw(rowstat - 1, startstat, msg);
          |                                          ^~~

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