perf_counter: add perf_print_counter_buffer & perf_iterate_all

These can be used to print the perf counters to a buffer.

src/platforms/ros/perf_counter.cpp

@@ -144,6 +144,16 @@ void		perf_print_counter_fd(int fd, perf_counter_t handle)
 
 }
 
+int		perf_print_counter_buffer(char *buffer, int length, perf_counter_t handle)
+{
+	return 0;
+}
+
+void	perf_iterate_all(perf_callback cb, void *user)
+{
+
+}
+
 /**
  * Print all of the performance counters.
  *