Fix implicit pointer to integral type cast

From gcc:

src/db_compact.c:87:5: warning: passing argument 3 of 'couchstore_compact_db_ex' makes integer from pointer without a cast [enabled by default]
src/db_compact.c:40:20: note: expected 'uint64_t' but argument is of type 'void *'

Change-Id: I9326c2bb4ff639fca995f1de4c0f5fe5172c11a1
Reviewed-on: http://review.couchbase.org/24162
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>