Add a workaround to avoid a crash produced by a GCC bug

This bug: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58800
in nth_element makes QtConcurrent::blockingFilter crash since underneath
QtConcurrent uses QtConcurrent::Median which uses nth_element.

We need the workaround so we can continue working on
kde-workspace/frameworks without having kded5 crashing.