plasma5: mark broken with Qt >= 5.15

pkgs/desktops/plasma-5/default.nix

@@ -36,6 +36,7 @@ let
   };
 
   mkDerivation = libsForQt5.callPackage ({ mkDerivation }: mkDerivation) {};
+  qtbase = libsForQt5.callPackage ({ qtbase }: qtbase) {};
 
   packages = self: with self;
     let
@@ -82,6 +83,7 @@ let
             setupHook = args.setupHook or defaultSetupHook;
 
             meta = {
+              broken = lib.versionAtLeast qtbase.version "5.15";
               license = with lib.licenses; [
                 lgpl21Plus lgpl3Plus bsd2 mit gpl2Plus gpl3Plus fdl12
               ];