Remove the useless platform guard

spec/ruby/core/file/utime_spec.rb

@@ -70,13 +70,11 @@ describe "File.utime" do
     end
   end
 
-  platform_is_not :windows do
-    it "sets nanosecond precision" do
-      t = Time.utc(2007, 11, 1, 15, 25, 0, 123456.789r)
-      File.utime(t, t, @file1)
-      File.atime(@file1).nsec.should.between?(0, 123500000)
-      File.mtime(@file1).nsec.should.between?(0, 123500000)
-    end
+  it "may set nanosecond precision" do
+    t = Time.utc(2007, 11, 1, 15, 25, 0, 123456.789r)
+    File.utime(t, t, @file1)
+    File.atime(@file1).nsec.should.between?(0, 123500000)
+    File.mtime(@file1).nsec.should.between?(0, 123500000)
   end
 
   platform_is :linux do