fixes #6049 add mixin release, acquire to withLock (#6113)

lib/core/locks.nim

@@ -59,6 +59,7 @@ proc signal*(cond: var Cond) {.inline.} =
 template withLock*(a: Lock, body: untyped) =
   ## Acquires the given lock, executes the statements in body and
   ## releases the lock after the statements finish executing.
+  mixin acquire, release
   a.acquire()
   {.locks: [a].}:
     try: