Concurency: add Windows.h include

The Windows path uses `LoadLibrary` which should use `Windows.h` for the
declaration as per MSDN.  This avoids an unnecessary warning when
building for Windows.

Author: compnerd

stdlib/public/Concurrency/DispatchGlobalExecutor.inc

@@ -24,7 +24,9 @@
 
 #if SWIFT_CONCURRENCY_ENABLE_DISPATCH
 #include <dispatch/dispatch.h>
-#if !defined(_WIN32)
+#if defined(_WIN32)
+#include <Windows.h>
+#else
 #include <dlfcn.h>
 #endif
 #endif