Fix missing import

'slime-switch-to-output-buffer'  and 'slime-repl-return' are defined
in slime-repl.el.

I got following byte compile warnings with original code.

```
In end of data:
eval-in-repl-slime.el:74:1:Warning: the following functions are not known to be defined:
    slime-switch-to-output-buffer, slime-repl-return
```