Emacs: Fix notmuch-message-summary-face definition

Emacs: Fix notmuch-message-summary-face definition

Emacs face definition forms are either

    ((DISPLAY . PLIST)
     (DISPLAY . PLIST))

or

    ((DISPLAY PLIST)   ;For backward compatibility.
     (DISPLAY PLIST))

Commit a2388bc56e55da5d5695816818274f8a84b0ed92 (2020-08-08) follows
neither of the correct formats. It defines:

    `((((class color) (background light))
       ,@(and (>= emacs-major-version 27) '(:extend t))
       (:background "#f0f0f0"))
      (((class color) (background dark))
       ,@(and (>= emacs-major-version 27) '(:extend t))
       (:background "#303030")))

which produces:

    ((DISPLAY
      :extend t (:background "#f0f0f0"))
     (DISPLAY
      :extend t (:background "#303030")))

And that is wrong format.

This change fixes the face definition form to produce:

    ((DISPLAY
      :extend t :background "#f0f0f0")
     (DISPLAY
      :extend t :background "#303030"))

which follows the (DISPLAY . PLIST) format (see above).