tools/testing/cxl: Require CONFIG_DEBUG_FS

The cxl_mem driver uses debugfs to support poison inject and clear.
Add debugfs to the list of required symbols so that cxl_test can
emulate those poison operations.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/4f3aab57fbf1cc3ccde2eb887c5d90566c8d0e90.1681874357.git.alison.schofield@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>