pmem: add maintainer for include/linux/pmem.h

The file include/linux/pmem.h was recently created to hold the PMEM API,
and is logically part of the PMEM driver.  Add an entry for this file to
MAINTAINERS.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>