core-spacemacs-buffer: fixed a bug

There is an unreported bug that when startup list is set to shown `recents-by-project`,
Emacs reports that `projectile-mode` is an unknown symbol.

This commit fixed this bug by checking the existence of `projectile-mode`, which is
automatically loaded during startup.

Author: lebensterben

core/core-spacemacs-buffer.el

@@ -1386,7 +1386,8 @@ SEQ, START and END are the same arguments as for `cl-subseq'"
           (spacemacs-buffer//insert-errors)
           (spacemacs-buffer//insert-warnings))
          ((eq el 'recents) (spacemacs-buffer//insert-recent-files list-size))
-         ((eq el 'recents-by-project)
+         ((and (eq el 'recents-by-project)
+	       (fboundp 'projectile-mode))
           (spacemacs-buffer//insert-recent-files-by-project list-size))
          ((eq el 'todos) (spacemacs-buffer//insert-todos list-size))
          ((eq el 'agenda) (spacemacs-buffer//insert-agenda list-size))