ueviewer: add missing deps (#400202)

pkgs/by-name/ue/ueviewer/package.nix

···
   fetchFromGitHub,
   fetchpatch,
   unstableGitUpdater,
+  libGL,
   libpng,
+  libX11,
   perl,
   SDL2,
   zlib,
···
       zlib
     ]
     ++ lib.optionals (!gccStdenv.hostPlatform.isDarwin) [
+      libGL
+      libX11
       SDL2
     ];