Unbreak building unit tests on Windows after r266595.

llvm-svn: 266614
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
index 20e0872..fbeabe0 100644
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -17,6 +17,7 @@
 #include "gtest/gtest.h"
 
 #ifdef LLVM_ON_WIN32
+#include "llvm/ADT/ArrayRef.h"
 #include <windows.h>
 #include <winerror.h>
 #endif