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/include/proto/dumpstats.h b/include/proto/dumpstats.h
index cbbb802..2bdc3b1 100644
--- a/include/proto/dumpstats.h
+++ b/include/proto/dumpstats.h
@@ -54,7 +54,7 @@
 #define STAT_SCOPE_INPUT_NAME "scope" /* pattern form scope name <input> in html form */
 #define STAT_SCOPE_PATTERN    "?" STAT_SCOPE_INPUT_NAME "="
 
-extern struct si_applet http_stats_applet;
+extern struct applet http_stats_applet;
 
 void stats_io_handler(struct stream_interface *si);