file.c: birthtime on Windows

* file.c (stat_birthtime): `ctime` is actually `created time` on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46049 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

file.c

@@ -791,6 +791,7 @@ stat_birthtime(struct stat *st)
     return rb_time_nano_new(ts->tv_sec, ts->tv_nsec);
 }
 #elif defined(_WIN32)
+# define stat_birthtime stat_ctime
 #else
 # undef HAVE_STAT_BIRTHTIME
 #endif