jaf: fix fprintf argument

[OPNA2608]

Fixes compile error with -Werror=format-security enabled:

../src/core/jaf/error.c: In function 'jaf_print_expression': ../src/core/jaf/error.c:174:17: error: format not a string literal and no format arguments [-Werror=format-security]
  174 |                 fprintf(out, jaf_op_to_string(expr->op));
      |                 ^~~~~~~

This is already done in the other 2 instances where jaf_op_to_string is called & subsequently fprintf'd:

https://github.com/nunuhara/alice-tools/blob/5aa59f45a41b19914f4fe2c55a3f1ccb706d98a7/src/core/jaf/error.c#L163-L165

[nunuhara]

Thanks!