Expose xmlBufShrink in the public tree API

As suggested by Andrew W. Nosenko:
Proposal: expose the new xmlBufShrink() to the "public" API for
compatibility with xmlBufUse().

Reason: the following scenario:

1. Read something into  xmlParserInputBuffer (e.g. using
xmlParserInputBufferRead())
2. Extract content through xmlBufContent()
3. Extract content length through xmlBufUse().  Result have type
'size_t'.
4. Use this content
5. Now, you need to shrink the buffer.  How to do it?  Doing that
through legacy xmlBufferShrink() is unsafe because it uses 'unsigned
int' and the whole point of introducing the new API was handling the
cases, when 'unsigned int' is not enough.  Therefore, need to use the
new xmlBufShrink().  But it is "private".

Therefore, I propose to expose the new xmlBufShrink() in the same way,
as xmlBufContent() and xmlBufUse() are exposed.