lisp-mode.el: Add defvar-keymap to lisp-imenu-generic-expression

* lisp/emacs-lisp/lisp-mode.el (lisp-imenu-generic-expression):
Add defvar-keymap.  We do not ignore (defvar-keymap FOO)
constructs in imenu as we do with (defvar FOO) since the former
constructs are generally not vacuous whereas the latter ones
often are.  (Bug#64831)

lisp/emacs-lisp/lisp-mode.el

@@ -134,7 +134,7 @@
 	 (purecopy (concat "^\\s-*("
 			   (regexp-opt
 			    '(;; Elisp
-                              "defconst" "defcustom"
+                              "defconst" "defcustom" "defvar-keymap"
                               ;; CL
                               "defconstant"
 			      "defparameter" "define-symbol-macro")