lisp/org-compat.el: Allow using imenu to visit non-leaf headlines

* lisp/org-compat.el (org-imenu-get-tree): Add the current headline to
the tree as a simple item even if it isn't a leaf.

With a file like this:

* headline 1
** headline 2

We currently produce an imenu tree that looks like this:

'(("headline 1" ("headline 2" . marker-2)))

imenu has no clue where "headline 1" is located and thus the user
can't navigate to it.  With this patch installed imenu knows where
non-leaf headlines are as the tree will now look like this:

'(("headline 1" . marker-1)
  ("headline 1" ("headline 2" . marker-2)))

Quirks:

With the default `imenu-flatten' value of nil, it is still impossible
to visit non-leaf headlines and no change is perceived.

Setting `imenu-flatten' to 'group works as expected with the quirk
that top level headlines don't end up in the group.

Ex:
* Headline 1
Group is "*"
Setting the group to "Headline 1" somehow might be nice but would
require upstream changes in imenu.
** Headline 2
Group is "Headline 1"
*** Headline 3
Group is "Headline 1:Headline 2"

Everything seems to work as expected when `imenu-flatten' is set to
'prefix or 'annotation.

Link: https://orgmode.org/list/CH3PR84MB34241FF78D2A1D8653FE6056C5C22@CH3PR84MB3424.NAMPRD84.PROD.OUTLOOK.COM

lisp/org-compat.el

@@ -1458,8 +1458,8 @@ This also applied for speedbar access."
 	   (let* ((m (point-marker))
 		  (item (propertize headline 'org-imenu-marker m 'org-imenu t)))
 	     (push m org-imenu-markers)
-	     (if (>= level last-level)
-		 (push (cons item m) (aref subs level))
+             (push (cons item m) (aref subs level))
+             (unless (>= level last-level)
 	       (push (cons item
 			   (cl-mapcan #'identity (cl-subseq subs (1+ level))))
 		     (aref subs level))