[concurrency] conform Deque._Storage to Sendable

Author: glessard

Sources/DequeModule/Deque+Sendable.swift

@@ -10,3 +10,5 @@
 //===----------------------------------------------------------------------===//
 
 extension Deque: @unchecked Sendable where Element: Sendable {}
+
+extension Deque._Storage: @unchecked Sendable where Element: Sendable {}