Enable -Wno-unused-parameter

Instead of adding __attribute__((unused)) to unused arguments, add the
-Wno-unused-parameter compiler flag.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

tests/prog_reconnect_timeout.c

@@ -83,7 +83,7 @@ void event_loop(struct iscsi_context *iscsi, struct client_state *state)
 }
 
 void logout_cb(struct iscsi_context *iscsi, int status,
-	       void *command_data _U_, void *private_data)
+	       void *command_data, void *private_data)
 {
 	struct client_state *state = (struct client_state *)private_data;