Remove use of -isystem for googletest

Use standard include_dirs and a proxy header to manually suppress
warning. This solves the problem of sign conversion warning that
fire only when rolling into chrome.
Follow-up CLs will try to remove -isystem also to other
buildtools. Limiting the scope of this for now.

Change-Id: I8bc7924a95c7dd73cad54f447c122bd339e4e2a4
Bug: 129130827
diff --git a/src/protozero/message_unittest.cc b/src/protozero/message_unittest.cc
index 8bffabc..d75dcd0 100644
--- a/src/protozero/message_unittest.cc
+++ b/src/protozero/message_unittest.cc
@@ -22,10 +22,10 @@
 #include <utility>
 #include <vector>
 
-#include <gtest/gtest.h>
 #include "perfetto/base/logging.h"
 #include "src/base/test/utils.h"
 #include "src/protozero/test/fake_scattered_buffer.h"
+#include "test/gtest_and_gmock.h"
 
 namespace protozero {