Isolate nullptr to grpc namespace when we must define it ourselves
Change the one non-namespace use of nullptr to NULL (used as an
argument to a C function anyway)
diff --git a/test/cpp/qps/timer.cc b/test/cpp/qps/timer.cc
index 07289f6..c1ba23d 100644
--- a/test/cpp/qps/timer.cc
+++ b/test/cpp/qps/timer.cc
@@ -52,7 +52,7 @@
 Timer::Result Timer::Sample() {
   struct rusage usage;
   struct timeval tv;
-  gettimeofday(&tv, nullptr);
+  gettimeofday(&tv, NULL);
   getrusage(RUSAGE_SELF, &usage);
 
   Result r;