skwikisource: Restrict uploads to sysops

https://sk.wikisource.org/w/index.php?title=Wikizdroje:Port%C3%A1l_komunity&oldid=5183#Znemo.C5.BEn.C4.9Bn.C3.AD_lok.C3.A1ln.C3.ADho_uploadu

Change-Id: Icb90f23152e2882e9e8b90082923735b9e4fe49f