Won't build on emacs 29 - linum-mode has been replaced

linum-mode has been replaced with display-line-numbers-mode [Emacs 29 Release Notes:linum](https://github.com/emacs-mirror/emacs/blob/c4e038c7be38b2e6cf2d2c7c39264f068f789c02/etc/NEWS.29#L494) linum-mode has enough issues that it was retired. Most users will not have it. A vanilla emacs 29 build of git-gutter errors out with ``` Lisp error: (void-variable global-linum-mode) ```