Cygwin: Update release note for 3.6.6

Author: tyan0

winsup/cygwin/release/3.6.6

@@ -28,3 +28,7 @@ Fixes:
 - Fix primary group override for non-domain users and users of a
   Microsoft account.
   Addresses: https://cygwin.com/pipermail/cygwin/2025-December/259068.html
+
+- Fix the deadlock between the parent and child processes, where both lock
+  the same file and attempt to close it simultaneously.
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-December/259187.html