Load 'cl-lib' for using 'cl-case'

I got following error when `magic-latex-buffer.el` is byte-compiled.
This patch fixes this byte-compile warnings. Please see this patch.

```
In ml/jit-prettifier:
magic-latex-buffer.el:630:31:Warning: `(95)' is a malformed function
magic-latex-buffer.el:630:31:Warning: `(94)' is a malformed function

In end of data:
magic-latex-buffer.el:680:1:Warning: the function `cl-case' is not known to be
    defined.
```