Add dummy stubs for GPG public functions to be compiled instead of
original code in case if support of GPG is disabled.
Need that to keep API backward compatibility.
Based on original code from file `ostree-gpg-verify-result.c`.
Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Closes: #1889
Approved by: cgwalters
Denis Pynkin