Fix windows sdk build.

Change-Id: I0af79c0ada8de360a50dbfdf561ca5597672b7c5
diff --git a/clang-host-build.mk b/clang-host-build.mk
index e116c02..2b42b39 100644
--- a/clang-host-build.mk
+++ b/clang-host-build.mk
@@ -17,7 +17,9 @@
 LLVM_ROOT_PATH := external/llvm
 include $(LLVM_ROOT_PATH)/llvm.mk
 
+ifneq ($(HOST_OS),windows)
 include external/libcxx/libcxx.mk
+endif
 
 ifneq ($(LLVM_HOST_BUILD_MK),)
 include $(LLVM_HOST_BUILD_MK)