Fix File#birthtime and File.birthtime on Windows

gistfile1.diff

Index: file.c
===================================================================
--- file.c	(revision 49353)
+++ file.c	(working copy)
@@ -2193,10 +2193,10 @@
  *
  *  _file_name_ can be an IO object.
  *
- *  Note that on Windows (NTFS), returns creation time (birth time).
- *
  *     File.birthtime("testfile")   #=> Wed Apr 09 08:53:13 CDT 2003
  *
+ *  If the platform doesn't have birthtime, raises NotImplementedError.
+ *
  */
 
 static VALUE
@@ -2221,10 +2221,10 @@
  *
  *  Returns the birth time for <i>file</i>.
  *
- *  Note that on Windows (NTFS), returns creation time (birth time).
- *
  *     File.new("testfile").birthtime   #=> Wed Apr 09 08:53:14 CDT 2003
  *
+ *  If the platform doesn't have birthtime, raises NotImplementedError.
+ *
  */
 
 static VALUE