File#birthtime depends on the kernel version on Linux

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67117 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/file/birthtime_spec.rb

@@ -50,9 +50,11 @@ describe "File#birthtime" do
     end
   end
 
-  platform_is :linux, :openbsd do
+  platform_is :openbsd do
     it "raises an NotImplementedError" do
       lambda { @file.birthtime }.should raise_error(NotImplementedError)
     end
   end
+
+  # TODO: depends on Linux kernel version
 end