target: make device_mutex and device_list static

Variables device_mutex and device_list static are local to the source,
so make them static.

Cleans up sparse warnings:
"symbol 'device_list' was not declared. Should it be static?"
"symbol 'device_mutex' 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>