sched_ext: Add missing static to scx_dump_data

scx_dump_data is only used inside ext.c but doesn't have static. Add it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202409070218.RB5WsQ07-lkp@intel.com/