Fix check for SCHED_RESET_ON_FORK feature in clean build

Configuration macro HAVE_SCHED_RESET_ON_FORK is not reflecting availability of SCHED_RESET_ON_FORK
in config-kwin.h because that feature is detected after file is already generated.

Fix this by moving config-kwin.h generation after feature detection.

Differential Revision: https://phabricator.kde.org/D22337