[fix warnings] undeclared functions in cuetext.c

There are a bunch of function calls in cuetext.c prior to or without declaration. This is not likely to cause bugs because the return value should always fit in EAX, and the stack should deal with everything else, but should clean up the compile by declaring these functions somewhere.

```
Warning	6	warning C4013: 'webvtt_create_timestamp_node' undefined; assuming extern returning int	c:\github\webvtt\src\libwebvtt\cuetext.c	269	1	libwebvtt
Warning	4	warning C4013: 'webvtt_create_text_node' undefined; assuming extern returning int	c:\github\webvtt\src\libwebvtt\cuetext.c	258	1	libwebvtt
Warning	5	warning C4013: 'webvtt_create_internal_node' undefined; assuming extern returning int	c:\github\webvtt\src\libwebvtt\cuetext.c	264	1	libwebvtt
Warning	7	warning C4013: 'webvtt_create_head_node' undefined; assuming extern returning int	c:\github\webvtt\src\libwebvtt\cuetext.c	674	1	libwebvtt
Warning	8	warning C4013: 'webvtt_attach_node' undefined; assuming extern returning int	c:\github\webvtt\src\libwebvtt\cuetext.c	743	1	libwebvtt
```