bug#42010: 28.0.50; "You should byte-compile Gnus" message for natively compiled gnus.el

[Doug Davis]

When running `gnus' with a native compiled Emacs build a message is
reported in the minibuffer "You should byte-compile Gnus" and `sit-for'
is called. I'm not sure what check is possible to see if a function is
natively compiled, but perhaps it should be added to this logic in
gnus.el:

  (unless (byte-code-function-p (symbol-function 'gnus))
    (message "You should byte-compile Gnus")
    (sit-for 2))