/usr/share/postgresql/10/extension/tcn.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 | # tcn extension
comment = 'Triggered change notifications'
default_version = '1.0'
module_pathname = '$libdir/tcn'
relocatable = true
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