BUILD: trace: make the lockon_ptr const to silence a warning without threads

I forgot to fix this one before pushing, despite my tests. lockon_ptr is
only used to compare pointers, it doesn't need to point to a writable
location. Without threads the atomic store is turned into an assignment
and rightfully complains.
diff --git a/include/types/trace.h b/include/types/trace.h
index 6d85f9c..e43cb88 100644
--- a/include/types/trace.h
+++ b/include/types/trace.h
@@ -142,7 +142,7 @@
 	struct sink *sink;       // where to send the trace
 	/* trace state part below */
 	enum trace_state state;
-	void *lockon_ptr;        // what to lockon when lockon is set
+	const void *lockon_ptr;  // what to lockon when lockon is set
 };
 
 #endif /* _TYPES_TRACE_H */