qt55.kdeFrameworks: mark broken

KDE Frameworks 5.26 requires Qt 5.6. Qt 5.6 is a designated LTS release;
only proprietary packages should use older versions.

(cherry picked from commit 16dafb018e)

pkgs/development/libraries/kde-frameworks/default.nix

@@ -40,6 +40,7 @@ let
       let
         inherit (args) name;
         inherit (srcs."${name}") src version;
+        qtVersion = (builtins.parseDrvName self.qtbase.name).version;
       in kdeDerivation (args // {
         name = "${name}-${version}";
         inherit src;
@@ -50,6 +51,7 @@ let
           ];
           platforms = lib.platforms.linux;
           homepage = "http://www.kde.org";
+          broken = builtins.compareVersions qtVersion "5.6.0" < 0;
         } // (args.meta or {});
       });