Fix warning of "undefining the allocator of T_DATA class"

[printfinn]

Hi there, not sure if this is related to the project or Ruby, I noticed a warning when using the example code in README, it seems related to the new implementation of Ruby after 3.2:

The warning is warning: undefining the allocator of T_DATA class Libsvm::Model, to reproduce:

irb --simple-prompt
>> require 'libsvm'
>> 
>> # This library is namespaced.
>> problem = Libsvm::Problem.new
>> parameter = Libsvm::SvmParameter.new
>> 
>> parameter.cache_size = 1 # in megabytes
>> 
>> parameter.eps = 0.001
>> parameter.c = 10
>> 
>> examples = [ [1,0,1], [-1,0,-1] ].map {|ary| Libsvm::Node.features(ary) }
>> labels = [1, -1]
>> 
>> problem.set_examples(labels, examples)
>> 
>> model = Libsvm::Model.train(problem, parameter)
>> 
>> pred = model.predict(Libsvm::Node.features(1, 1, 1))
>> puts "Example [1, 1, 1] - Predicted #{pred}"
(irb):17: warning: undefining the allocator of T_DATA class Libsvm::Model
Example [1, 1, 1] - Predicted 1.0
=> nil
>> RUBY_VERSION
=> "3.4.1"

I did some research, found a potential solution, but I'm not very experienced in Ruby-C bindings. If experienced dev have time, please review the change.