MINOR: spoa_example: Count the number of frames processed by each worker

This is done for debug purpose. This way, it is easy to know if the load is
equally distributed between workers.
diff --git a/contrib/spoa_example/spoa.c b/contrib/spoa_example/spoa.c
index d8defd1..026f256 100644
--- a/contrib/spoa_example/spoa.c
+++ b/contrib/spoa_example/spoa.c
@@ -150,6 +150,7 @@
 	struct list         clients;
 
 	struct list         frames;
+	unsigned int        nbframes;
 };
 
 
@@ -1287,7 +1288,7 @@
 {
 	struct worker *worker = arg;
 
-	LOG(worker, "%u clients connected", worker->nbclients);
+	LOG(worker, "%u clients connected (%u frames)", worker->nbclients, worker->nbframes);
 }
 
 static void
@@ -1330,7 +1331,7 @@
 				goto stop_processing;
 			if (spoe_decode_data(&p, end, &data, &type) == -1)
 				goto skip_message;
-
+			frame->worker->nbframes++;
 			if (type == SPOE_DATA_T_IPV4)
 				check_ipv4_reputation(frame, &data.ipv4);
 			if (type == SPOE_DATA_T_IPV6)
@@ -1819,6 +1820,7 @@
 
 		w->id        = i+1;
 		w->nbclients = 0;
+		w->nbframes  = 0;
 		LIST_INIT(&w->engines);
 		LIST_INIT(&w->clients);
 		LIST_INIT(&w->frames);