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 <[e-mail]>
Signed-off-by: Santosh Sivaraj <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

tools/testing/nvdimm/Kbuild

···
 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