kdeApps_15_04.okular: enable libspectre for PostScript

pkgs/applications/kde-apps-15.04/default.nix

···
         buildInputs = super.libksane.buildInputs ++ [scope.KDE4 saneBackends];
       };
 
+      okular = super.okular // {
+        nativeBuildInputs =
+          super.okular.nativeBuildInputs
+          ++ [ pkgs.pkgconfig ];
+      };
+
       rocs = super.rocs // {
         buildInputs = super.rocs.buildInputs ++ (with kf5; [ kdelibs4support ]);
       };