[warning] Pointless 'const' attribute to 'uint32' return values in webvtt/string.h

webvtt/string.h ( lines 124 - 136 )
```
/**
 * webvtt_string_length
 *
 * return the length of a strings text
*/
WEBVTT_EXPORT const webvtt_uint32 webvtt_string_length( const webvtt_string *str );

/**
 * webvtt_string_capacity
 *
 * return the current capacity of a string
 */
WEBVTT_EXPORT const webvtt_uint32 webvtt_string_capacity( const webvtt_string *str );
```

`const webvtt_uint32` does not really make sense, as return values go. Remove `const` attribute.