Enable building gtest for Windows

Change-Id: Ib4a865ca7716084498eff32e057865c0ffbced46
diff --git a/build/windows_sdk_whitelist.mk b/build/windows_sdk_whitelist.mk
index dfeaf3a..d91dde5 100644
--- a/build/windows_sdk_whitelist.mk
+++ b/build/windows_sdk_whitelist.mk
@@ -40,6 +40,7 @@
 	external/clang \
 	external/easymock \
 	external/expat \
+	external/gtest \
 	external/libcxx \
 	external/libcxxabi \
 	external/compiler-rt \