Allow to build "winsdk-tools" alone.

Allow to run "make winsdk-tools" without building the full win_sdk.

Bug: 19149444
Change-Id: If98495035f4f84dbcab7af2a74434e589bbfd4d1
diff --git a/core/Makefile b/core/Makefile
index 49f48fc..6bafdb9 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1719,7 +1719,7 @@
 MAIN_SDK_NAME := $(sdk_name)
 MAIN_SDK_DIR  := $(sdk_dir)
 MAIN_SDK_ZIP  := $(INTERNAL_SDK_TARGET)
-ifneq ($(filter win_sdk,$(MAKECMDGOALS)),)
+ifneq ($(filter win_sdk winsdk-tools,$(MAKECMDGOALS)),)
 include $(TOPDIR)development/build/tools/windows_sdk.mk
 endif