Fix compilation with ruby-3.4 and gcc-14

[mvidner]

Depending on the numeric argument, the function pointer passed is expected to have different signatures. Up go gcc-13, it is only a warning (and the called method would have raised anyway), but with gcc-14 it becomes a compilation error.

See also: https://docs.ruby-lang.org/en/master/extension_rdoc.html#label-Method+and+singleton+method+definition