/usr/share/postgresql/10/extension/dblink.control is in postgresql-10 10.6-0ubuntu0.18.04.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | # dblink extension
comment = 'connect to other PostgreSQL databases from within a database'
default_version = '1.2'
module_pathname = '$libdir/dblink'
relocatable = true
|