[spacemacs-misc] Ensure that dumb-jump xref backend is actually last

The comment suggests the intention of the code is to add it to the
end, but it wasn't doing so (and examining the xref-backend-functions
of a running Spacemacs shows that dumb-jump-xref-activate is first).

layers/+spacemacs/spacemacs-misc/packages.el

@@ -41,7 +41,7 @@
 
     ;; Enable xref-backend of dumb-jump. It's chosen only when no better
     ;; options is available
-    (add-hook 'xref-backend-functions #'dumb-jump-xref-activate)))
+    (add-hook 'xref-backend-functions #'dumb-jump-xref-activate 90)))
 
 (defun spacemacs-misc/init-request ()
   (setq request-storage-directory