nvmet: Make nvmet_debugfs static

The sparse tool complains as follows:

drivers/nvme/target/debugfs.c:16:15: warning:
	symbol 'nvmet_debugfs' was not declared. Should it be static?

This symbol is not used outside debugfs.c, so marks it static.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>