Add Fabien to the keys for gitian signing

Summary: As per title

Test Plan:

./check-keys.sh

Ensure all the keys are already here or imported

Note: I cannot reach the current keyserver in check-keys.sh from my machine atm.
If the script fails for you, consider patching D2420 first.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2421