tcmu: make array tcmu_attrib_attrs static const

The array tcmu_attrib_attrs does not need to be in global scope, so make
it static.

Cleans up sparse warning:
"symbol 'tcmu_attrib_attrs' was not declared. Should it be static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>