sxiv: add h7x4 as maintainer

pkgs/applications/graphics/sxiv/default.nix

···
     homepage = "https://github.com/muennich/sxiv";
     license = lib.licenses.gpl2Plus;
     platforms = lib.platforms.linux;
-    maintainers = with maintainers; [ ];
+    maintainers = with maintainers; [ h7x4 ];
   };
 }