tools/test/nvdimm: Fix out of tree build

tools/test/nvdimm: Fix out of tree build

Out of tree build using

   make M=tools/test/nvdimm O=/tmp/build -C /tmp/build

fails with the following error

make: Entering directory '/tmp/build'
  CC [M]  tools/testing/nvdimm/test/nfit.o
linux/tools/testing/nvdimm/test/nfit.c:19:10: fatal error: nd-core.h: No such file or directory
   19 | #include <nd-core.h>
      |          ^~~~~~~~~~~
compilation terminated.

That is because the kbuild file uses $(src) which points to
tools/testing/nvdimm, $(srctree) correctly points to root of the linux
source tree.

Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Santosh Sivaraj <santosh@fossix.org>
Link: https://lore.kernel.org/r/20200114054051.4115790-1-santosh@fossix.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>