tools/testing/nvdimm: Fix missing 'fallthrough' warning

Use "fallthrough;" to address:

tools/testing/nvdimm/test/nfit.c: In function ‘nd_intel_test_finish_query’:
tools/testing/nvdimm/test/nfit.c:436:37: warning: this statement may
	fall through [-Wimplicit-fallthrough=]
  436 |                 fw->missed_activate = false;
      |                 ~~~~~~~~~~~~~~~~~~~~^~~~~~~
tools/testing/nvdimm/test/nfit.c:438:9: note: here
  438 |         case FW_STATE_UPDATED:
      |         ^~~~

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Link: https://lore.kernel.org/r/162767522046.3313209.14767278726893995797.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>