IO::Buffer.map seems not shareable across processes on OpenBSD

* See https://rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20260129T163005Z.fail.html.gz

Author: eregon

spec/ruby/core/io/buffer/map_spec.rb

@@ -73,7 +73,7 @@ def open_big_file_fixture
     @buffer.should.valid?
   end
 
-  platform_is_not :windows do
+  platform_is_not :windows, :openbsd do
     it "is shareable across processes" do
       file_name = tmp("shared_buffer")
       @file = File.open(file_name, "w+")