Add plugin periodic function elisp skeleton

Change-Id: Ide4e2d2a06dff20c94ae5436ba6361b246052867
Signed-off-by: Dave Barach <dave@barachs.net>
diff --git a/extras/emacs/plugin-am-skel.el b/extras/emacs/plugin-am-skel.el
index 5a082b4..0a218e2 100644
--- a/extras/emacs/plugin-am-skel.el
+++ b/extras/emacs/plugin-am-skel.el
@@ -41,6 +41,7 @@
 " plugin-name "_plugin_la_SOURCES = \\
   " plugin-name "/node.c \\
   " plugin-name "/" plugin-name ".c \\
+  " plugin-name "/" plugin-name "_periodic.c \\
   " plugin-name "/" plugin-name ".h \\
   " plugin-name "/" plugin-name "_all_api_h.h \\
   " plugin-name "/" plugin-name "_msg_enum.h