Remove final third_party/gtest reference.

Missed this in https://github.com/bazelbuild/bazel/commit/8e9f4a8591d65c7972aea3957c57601570e0a39b.

RELNOTES: None.
PiperOrigin-RevId: 190506543
diff --git a/src/test/cpp/util/windows_test_util_test.cc b/src/test/cpp/util/windows_test_util_test.cc
index e408246..d777a91 100644
--- a/src/test/cpp/util/windows_test_util_test.cc
+++ b/src/test/cpp/util/windows_test_util_test.cc
@@ -15,7 +15,7 @@
 
 #include <string>
 
-#include "gtest/gtest.h"
+#include "googletest/include/gtest/gtest.h"
 #include "src/test/cpp/util/windows_test_util.h"
 
 #if !defined(COMPILER_MSVC) && !defined(__CYGWIN__)