MINOR: trace: add allocation of buffer-sized trace buffers

This will be needed so that we can implement protocol decoders which will
have to emit their contents into such a buffer.
diff --git a/include/proto/trace.h b/include/proto/trace.h
index 0838551..a299237 100644
--- a/include/proto/trace.h
+++ b/include/proto/trace.h
@@ -32,6 +32,7 @@
 #include <types/trace.h>
 
 extern struct list trace_sources;
+extern THREAD_LOCAL struct buffer trace_buf;
 
 /* registers trace source <source>. Modifies the list element!
  * The {start,pause,stop,report} events are not changed so the source may