Add deleted file back

Author: Catfish-Man

Sources/CSystem/shims.c

@@ -0,0 +1,18 @@
+/*
+ This source file is part of the Swift System open source project
+
+ Copyright (c) 2020 Apple Inc. and the Swift System project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+*/
+
+#ifdef __linux__
+
+#include <CSystemLinux.h>
+
+#endif
+
+#if defined(_WIN32)
+#include <CSystemWindows.h>
+#endif