qemu-iotests: make 218 executable

Commit 990dc39c made all tests executable at the time, but 218 came in
later, and missing those permissions.
Signed-off-by: NCleber Rosa <crosa@redhat.com>
Message-Id: <20181004161852.11673-4-crosa@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>