target_core_sbc: use noop for SYNCHRONIZE_CACHE

Windows does not expect SYNCHRONIZE_CACHE to be not supported,
and will generate a BSOD upon shutdown when using rd_mcp backend.
So better use a noop here.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>