nmixer: 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:

nmixer.cc:219:26: error: format not a string literal and no format arguments [-Werror=format-security]
  219 |                 mvwprintw(mixwin, my_y - 1, my_x, (char*)source);
      |                 ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

[Mart-Bogdan]

:+1: for this one. I have encountered this problem when tried to install it with new GCC.