Add tvb_ensure_bytes_exist64 to the API list.

I thought ws_symbol_export.h was supposed to eliminate API list files.
Apparently it didn't, so we have to update this file every time we add a
new API.

Change-Id: I93dda6ccc992c79d52d2b460e818995e6332f1be
Reviewed-on: https://code.wireshark.org/review/3363
Reviewed-by: Guy Harris <guy@alum.mit.edu>