Fix bug in prelude-kill-whole-line with prefix argument

`prelude-kill-whole-line` would error out with `prelude-kill-whole-line: Wrong type argument: number-or-marker-p, (4)` when passing it a <kbd>c-u</kbd> argument (i.e. <kbd>c-u</kbd> <kbd>s-k</kbd>).

The fix is to use `(interactive "p")` in the wrapper function `prelude-kill-whole-line`, just as `kill-whole-line` does in `simple.el`.

From `interactive` help page:
```
p -- Prefix arg converted to number.  Does not do I/O.
P -- Prefix arg in raw form.  Does not do I/O.
```