libnvdimm, namespace: make a couple of functions static

The functions create_namespace_pmem and create_namespace_blk are local
to the source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
symbol 'create_namespace_pmem' was not declared. Should it be static?
symbol 'create_namespace_blk' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>