clangd: Change Windows.h to windows.h.

This makes the file more cross compilation friendly.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@358549 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/clangd/Threading.cpp b/clangd/Threading.cpp
index 733267c..75b691b 100644
--- a/clangd/Threading.cpp
+++ b/clangd/Threading.cpp
@@ -10,7 +10,7 @@
 #elif defined(__APPLE__)
 #include <sys/resource.h>
 #elif defined (_WIN32)
-#include <Windows.h>
+#include <windows.h>
 #endif
 
 namespace clang {