python3Packages.ueberzug: limit to Linux

Starting with 18.2.0, one of ueberzug's headers #errors if __linux__
isn't defined [1]. Set meta.platforms to reflect this.

[1]: https://github.com/ueber-devel/ueberzug/blob/c7c151aec4fe31574a5a42b8b18eb5f80212ac00/ueberzug/X/python.h#L4-L6

pkgs/development/python-modules/ueberzug/default.nix

···
 
   pythonImportsCheck = [ "ueberzug" ];
 
-  meta = with lib; {
+  meta = {
     description = "Alternative for w3mimgdisplay";
     homepage = "https://github.com/ueber-devel/ueberzug";
     changelog = "https://github.com/ueber-devel/ueberzug/releases/tag/${version}";
-    license = licenses.gpl3Only;
+    license = lib.licenses.gpl3Only;
     mainProgram = "ueberzug";
-    maintainers = with maintainers; [ Br1ght0ne ];
+    maintainers = with lib.maintainers; [ Br1ght0ne ];
+    platforms = lib.platforms.linux;
   };
 }