Remove [extern_c] attribute from WinSDK module.

I've verified that all of the header files in this module already
contain their own `extern "C"` blocks that are activated if compiling as
C++.

The additional [extern_c] attribute causes problems for some headers,
like PathCch.h, that define additional overloads of functions when
compiled as C++. The "global" [extern_c] essentially switches off name
mangling with these overloads, which causes them to conflict with the
functions they are overloading.

See here for more context:
swiftlang#30233 (comment)

See here for an example of failures caused by the [extern_c]:
https://ci-external.swift.org/job/swift-PR-windows/869/console

(Search for "PathCch.h".)

Author: martinboehme

stdlib/public/Platform/winsdk.modulemap

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-module WinSDK [system] [extern_c] {
+module WinSDK [system] {
   module WinSock2 {
     header "WinSock2.h"
     header "WS2tcpip.h"