change gensym to cl-gensym

[pacew]

I encountered emacs-ctable as in indirect dependency of emacs-jedi, and it was causing an error message:

Eager macro-expansion failure: (void-function gensym)

This error can be reproduced with

M-x package-install RET ctable RET
(require 'ctable)

(I got ctable-20210128.629) Patching .emacs.d/elpa/ctable-20210128.629/ctable.el as in this PR made (require 'ctable) work without error.