Fix a method redefinition warning

[Earlopain]

$ bundle exec rspec -w
/home/user/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/ffi-1.16.3/lib/ffi/function.rb:65: warning: method redefined; discarding old vips_object_print_all
/home/user/rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/ffi-1.16.3/lib/ffi/function.rb:65: warning: method redefined; discarding old vips_object_print_all
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117 examples, 0 failures

This warning also appears when running tests in projects that make use of ruby-vips.

Simply remove the first definition. Note: attach_function does not respect visibility modifiers. The methods in the private section are in fact public. So there is no behaviour change here.

[jcupitt]

🤦 thanks for spotting this!