libpq: Add PQfullProtocolVersion to exports.txt

This is necessary to be able to actually use the function on Windows;
bug introduced in commit cdb6b0f.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQTfc_O+HXqAo5_-xG4r3EFVsTefUeQzSvhEyyLDba-O9w@mail.gmail.com

Author: robertmhaas

src/interfaces/libpq/exports.txt

@@ -209,3 +209,4 @@ PQservice                 206
 PQsetAuthDataHook         207
 PQgetAuthDataHook         208
 PQdefaultAuthDataHook     209
+PQfullProtocolVersion     210