editors/emacs: Fix native compilation with canna flavor

isearch-ext.el is a file that comes with the canna patch. Because it is
preloaded, it was native-compiled during the bootstrap stage. This
caused the native-lisp directory to be created, which in turn meant AOT
for all other elisp was silently skipped.

Fix this by adding isearch-ext to native-comp-bootstrap-deny-list, so it
is native-compiled later along with the rest of the elisp.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit f9566b80622d676208b0f0c5b4ed6f286ce37b10)