fix compiler error in vbucket.cc metaDataDisk +=

Change-Id: I5b2992d3298d5b65f959f48cb078ed1c1f7a01b1
Reviewed-on: http://review.couchbase.org/40150
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>