CLEANUP: applet: rename struct si_applet to applet

Since this one does not depend on stream_interface anymore, remove the
"si_" prefix.
diff --git a/src/peers.c b/src/peers.c
index 7683150..2a927e0 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1055,7 +1055,7 @@
 	goto out;
 }
 
-static struct si_applet peer_applet = {
+static struct applet peer_applet = {
 	.obj_type = OBJ_TYPE_APPLET,
 	.name = "<PEER>", /* used for logging */
 	.fct = peer_io_handler,