port r15756 from 1.2: Fix warning

src/debug/traces/starpu_fxt.c

@@ -949,8 +949,6 @@ static void handle_codelet_details(struct fxt_ev_64 *ev STARPU_ATTRIBUTE_UNUSED,
 	unsigned sched_ctx = ev->param[1];
 	if (worker < 0) return;
 
-	char *prefix = options->file_prefix;
-
 	int i;
 	char parameters[256];
 	size_t eaten = 0;
@@ -971,6 +969,8 @@ static void handle_codelet_details(struct fxt_ev_64 *ev STARPU_ATTRIBUTE_UNUSED,
 	{
 
 #ifdef STARPU_ENABLE_PAJE_CODELET_DETAILS
+		char *prefix = options->file_prefix;
+
 		worker_set_detailed_state(last_codelet_start[worker], prefix, worker, last_codelet_symbol[worker], ev->param[2], parameters, ev->param[3], ev->param[4], job_id);
 		if (sched_ctx != 0)
 		{