Use birthtimeMs, not ctimeMs (#1220)

The c in ctime means changed, not created.
diff --git a/okio-nodefilesystem/src/main/kotlin/okio/NodeJsFileSystem.kt b/okio-nodefilesystem/src/main/kotlin/okio/NodeJsFileSystem.kt
index 433d7f7..2c7cf41 100644
--- a/okio-nodefilesystem/src/main/kotlin/okio/NodeJsFileSystem.kt
+++ b/okio-nodefilesystem/src/main/kotlin/okio/NodeJsFileSystem.kt
@@ -63,7 +63,7 @@
       isDirectory = (stat.mode.toInt() and S_IFMT) == S_IFDIR,
       symlinkTarget = symlinkTarget,
       size = stat.size.toLong(),
-      createdAtMillis = stat.ctimeMs.toLong(),
+      createdAtMillis = stat.birthtimeMs.toLong(),
       lastModifiedAtMillis = stat.mtimeMs.toLong(),
       lastAccessedAtMillis = stat.atimeMs.toLong(),
     )