Fix the CI failure in OpenBSD

LibreSSL seems not to support `scrypt`.

https://rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20231105T233005Z.fail.html.gz
```
1)
OpenSSL::KDF.scrypt creates the same value with the same input ERROR
NoMethodError: undefined method `scrypt' for module OpenSSL::KDF
```

spec/ruby/library/openssl/kdf/scrypt_spec.rb

@@ -1,7 +1,7 @@
 require_relative '../../../spec_helper'
 require 'openssl'
 
-guard -> { OpenSSL::OPENSSL_VERSION_NUMBER >= 0x10100000 } do
+guard -> { OpenSSL::KDF.respond_to?(:scrypt) } do
   describe "OpenSSL::KDF.scrypt" do
     before :each do
       @defaults = {