Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/rsCppUtils.h b/rsCppUtils.h
index 7432109..7f1d58c 100644
--- a/rsCppUtils.h
+++ b/rsCppUtils.h
@@ -78,7 +78,7 @@
     // we don't support the clocks here.
     struct timeval t;
     t.tv_sec = t.tv_usec = 0;
-    gettimeofday(&t, NULL);
+    gettimeofday(&t, nullptr);
     return nsecs_t(t.tv_sec)*1000000000LL + nsecs_t(t.tv_usec)*1000LL;
 #endif
 }