Prevent a warning of "a candidate for gnu_printf format attribute"

[mame]

GCC 13 prints the following warning.

https://rubyci.s3.amazonaws.com/ubuntu/ruby-master/log/20241127T001003Z.log.html.gz

compiling generator.c
generator.c: In function ‘raise_generator_error’:
generator.c:91:5: warning: function ‘raise_generator_error’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
   91 |     VALUE str = rb_vsprintf(fmt, args);
      |     ^~~~~

This change prevents the warning by specifying the format attribute.