tools/testing/nvdimm: Fix compilation failure without CONFIG_DEV_DAX_PMEM_COMPAT

When a kernel is configured without CONFIG_DEV_DAX_PMEM_COMPAT, the
compilation of tools/testing/nvdimm fails with:

  Building modules, stage 2.
  MODPOST 11 modules
ERROR: "dax_pmem_compat_test" [tools/testing/nvdimm/test/nfit_test.ko] undefined!

Fix the problem by calling dax_pmem_compat_test() only if the kernel has
the required functionality.
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200123154720.12097-1-jack@suse.czSigned-off-by: Dan Williams <dan.j.williams@intel.com>

tools/testing/nvdimm/test/nfit.c

@@ -3164,7 +3164,9 @@ static __init int nfit_test_init(void)
 	mcsafe_test();
 	dax_pmem_test();
 	dax_pmem_core_test();
+#ifdef CONFIG_DEV_DAX_PMEM_COMPAT
 	dax_pmem_compat_test();
+#endif
 
 	nfit_test_setup(nfit_test_lookup, nfit_test_evaluate_dsm);