Fix Libc++ build with MinGW64

Summary: This patch corrects the build errors I encountered when building on MinGW64.

Reviewers: mati865, rnk, compnerd, smeenai, bcraig

Reviewed By: mati865, smeenai

Subscribers: martell, chapuni, cfe-commits

Differential Revision: https://reviews.llvm.org/D33082

llvm-svn: 304360
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 1ec026252b3027f06c8a561d93fc0a3801107bba
diff --git a/src/thread.cpp b/src/thread.cpp
index 412a8fa..550da8e 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -34,7 +34,7 @@
 
 #if defined(_LIBCPP_WIN32API)
 #include <windows.h>
-#endif // defined(_LIBCPP_WIN32API)
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD