Increase serialize buffer size from 512 to 768 bytes.

Now all the tests can run properly again. The latest fridge status
message did not fit in the 512 bytes all the way.

Change-Id: I087a46af71394d3e80f6a95f89601f71da092a01
diff --git a/aos/common/queue_types.cc b/aos/common/queue_types.cc
index 29f575c..9e9505d 100644
--- a/aos/common/queue_types.cc
+++ b/aos/common/queue_types.cc
@@ -409,7 +409,7 @@
         current = current->next;
       }
     }
-    char buffer[512];
+    char buffer[768];
     ssize_t size = cached.type.Serialize(buffer, sizeof(buffer));
     if (size == -1) {
       LOG(FATAL, "type %s is too big to fit into %zd bytes\n",