Fix aos_dump not counting messages bug

Rakhi reports that the --count argument was being ignored.  Turns out we
weren't counting anything...

Change-Id: I995821a510e9b8c00431ab5d6cd8d6ab7e74c780
diff --git a/aos/aos_dump.cc b/aos/aos_dump.cc
index a110fce..8e4c6f3 100644
--- a/aos/aos_dump.cc
+++ b/aos/aos_dump.cc
@@ -234,6 +234,7 @@
             const aos::Context &context, const void * /*message*/) {
           if (context.monotonic_event_time > next_send_time) {
             PrintMessage(channel, context, &str_builder);
+            ++message_count;
             next_send_time = context.monotonic_event_time +
                              std::chrono::milliseconds(FLAGS_rate_limit);
             if (FLAGS_count > 0 && message_count >= FLAGS_count) {