Add Windows guard

* It seems Windows only has microseconds resolution.

Author: eregon

core/file/utime_spec.rb

@@ -70,11 +70,13 @@
     end
   end
 
-  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 == 123456789
-    File.mtime(@file1).nsec.should == 123456789
+  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 == 123456789
+      File.mtime(@file1).nsec.should == 123456789
+    end
   end
 
   platform_is :linux do