vnc: fix no-lock-key-sync strncmp() length

The no-lock-key-sync option is being parsed incorrectly because of an
outdated strcmp() length value.  Use the correct length so that invalid
option names do not match.

Reported-by: Dr David Alan Gilbert <davidagilbert@uk.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
