Require 'org-clock

This is needed for 'org-clocking-p, as introduced in #3.

The commit therefore avoids the byte-compilation warning:

```
org-pomodoro.el:330:1:Warning: the function `org-clocking-p' is not known to be defined.
```

There's also `org-clock-is-active` in `org.el`, which sounds like it should give the same results, but its implementation is different. You might prefer to use that instead, if it turns out to do the same thing.