MAINTAINERS: clarify maintenance of nvdimm testing tool

The git history shows that the files under ./tools/testing/nvdimm are
being developed and maintained by the LIBNVDIMM maintainers.

This was identified with a small script that finds all files only
belonging to "THE REST" according to the current MAINTAINERS file, and I
acted upon its output.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20200201170933.924-1-lukas.bulwahn@gmail.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

MAINTAINERS

@@ -9569,6 +9569,7 @@ F:	drivers/acpi/nfit/*
 F:	include/linux/nd.h
 F:	include/linux/libnvdimm.h
 F:	include/uapi/linux/ndctl.h
+F:	tools/testing/nvdimm/
 
 LICENSES and SPDX stuff
 M:	Thomas Gleixner <tglx@linutronix.de>