ikiwiki: mark build broken

The test suite fails after we updated to Imagemagick from version 6.9.7-6 to
6.9.8-4. https://ikiwiki.info/bugs/imagemagick_6.9.8_test_suite_failure/ has
more details.

pkgs/applications/misc/ikiwiki/default.nix

···
     description = "Wiki compiler, storing pages and history in a RCS";
     homepage = http://ikiwiki.info/;
     license = stdenv.lib.licenses.gpl2Plus;
-
     platforms = stdenv.lib.platforms.linux;
     maintainers = [ stdenv.lib.maintainers.peti ];
+    broken = true; # https://ikiwiki.info/bugs/imagemagick_6.9.8_test_suite_failure/
   };
 }