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

tools/testing/nvdimm/Kbuild

@@ -21,8 +21,8 @@ DRIVERS := ../../../drivers
 NVDIMM_SRC := $(DRIVERS)/nvdimm
 ACPI_SRC := $(DRIVERS)/acpi/nfit
 DAX_SRC := $(DRIVERS)/dax
-ccflags-y := -I$(src)/$(NVDIMM_SRC)/
-ccflags-y += -I$(src)/$(ACPI_SRC)/
+ccflags-y := -I$(srctree)/drivers/nvdimm/
+ccflags-y += -I$(srctree)/drivers/acpi/nfit/
 
 obj-$(CONFIG_LIBNVDIMM) += libnvdimm.o
 obj-$(CONFIG_BLK_DEV_PMEM) += nd_pmem.o

tools/testing/nvdimm/test/Kbuild

 # SPDX-License-Identifier: GPL-2.0
-ccflags-y := -I$(src)/../../../../drivers/nvdimm/
-ccflags-y += -I$(src)/../../../../drivers/acpi/nfit/
+ccflags-y := -I$(srctree)/drivers/nvdimm/
+ccflags-y += -I$(srctree)/drivers/acpi/nfit/
 
 obj-m += nfit_test.o
 obj-m += nfit_test_iomap.o