checkpatch: add __pmem to $Sparse annotations

commit 61031952

 ("arch, x86: pmem api for ensuring durability of
persistent memory updates") added a new __pmem annotation for sparse
verification.  Add __pmem to the $Sparse variable so checkpatch can
appropriately ignore uses of this attribute too.

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>