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>

MAINTAINERS

@@ -6134,6 +6134,7 @@ L:	linux-nvdimm@lists.01.org
 Q:	https://patchwork.kernel.org/project/linux-nvdimm/list/
 S:	Supported
 F:	drivers/nvdimm/pmem.c
+F:	include/linux/pmem.h
 
 LINUX FOR IBM pSERIES (RS/6000)
 M:	Paul Mackerras <paulus@au.ibm.com>