Explicitly add a(n empty) keymap for Flymake

Too early to decide what will be in it, if anything.  Though "M-n" and
"M-p" would be great.

* lisp/progmodes/flymake-ui.el (flymake-mode-map): New variable

lisp/progmodes/flymake.el

@@ -631,9 +631,14 @@ backends."
         (add-hook 'post-command-hook #'start 'append 'local)
       (start))))
 
+(defvar flymake-mode-map
+  (let ((map (make-sparse-keymap)))
+    map)
+  "Keymap for `flymake-mode'.")
+
 ;;;###autoload
 (define-minor-mode flymake-mode nil
-  :group 'flymake :lighter flymake--mode-line-format
+  :group 'flymake :lighter flymake--mode-line-format :keymap flymake-mode-map
   (setq flymake--running-backends nil
         flymake--disabled-backends nil)
   (cond