THANKS: Add contributors from gthread improvements PRs

Add contributors from the incorporated PRs:
- Dominik Działak (PR #3308)
- Oliver Allen (PR #3303)
- sylt (PR #3157)

THANKS

@@ -58,6 +58,7 @@ Diego Oliveira <contact@diegoholiveira.com>
 Dima Barsky <github@kappa.ac93.org>
 Djoume Salvetti <djoume@freshbooks.com>
 Dmitry Medvinsky <me@dmedvinsky.name>
+Dominik Działak <ddzialak@users.noreply.github.com>
 Dustin Ingram <di@users.noreply.github.com>
 Ed Morley <edmorley@users.noreply.github.com>
 Eric Florenzano <floguy@gmail.com>
@@ -136,6 +137,7 @@ Neil Williams <neil@reddit.com>
 Nick Pillitteri <nick@tshlabs.org>
 Nik Nyby <nnyby@columbia.edu>
 Nikolay Kim <fafhrd91@gmail.com>
+Oliver Allen <oallenj@users.noreply.github.com>
 Oliver Bristow <evilumbrella+github@gmail.com>
 Oliver Tonnhofer <olt@bogosoft.com>
 Omer Katz <omer.drow@gmail.com>
@@ -168,6 +170,7 @@ Stephane Wirtel <stephane@wirtel.be>
 Stephen DiCato <Locker537@gmail.com>
 Stephen Holsapple <sholsapp@gmail.com>
 Steven Cummings <estebistec@gmail.com>
+sylt <sylt@users.noreply.github.com>
 Sébastien Fievet <zyegfryed@gmail.com>
 Tal Einat <532281+taleinat@users.noreply.github.com>
 Talha Malik <talham7391@hotmail.com>