MINOR: traces: always mark trace_source as thread-aligned

Some perf profiles occasionally show that reading the trace source's
state can take some time, which is not expected at all. It just happens
that the trace_source is not cache-aligned so depending on linkage, it
may share a cache line with a more active variable, thereby inducing a
slow down to all threads trying to read the variable.

Let's always mark it aligned to avoid this. For now the problem was not
observed again.

include/haproxy/trace-t.h

@@ -192,7 +192,7 @@ struct trace_source {
 	const void *lockon_ptr;  // what to lockon when lockon is set
 	const struct trace_source *follow; // other trace source's tracker to follow
 	int cmdline;             // true if source was activated via -dt command line args
-};
+} THREAD_ALIGNED();
 
 #endif /* _HAPROXY_TRACE_T_H */