Fix compatibility with BIND >= 9.10.4b1: hashsize() return type changed
Since bind version 9.10.4b1 (libdns version 164), the return type of
hashsize() has changed from unsigned int to size_t. Without this change
the plugin does not compile against bind 9.10.4b1 or newer on 64bit
architecture.
Signed-off-by: Tomas Hozza <thozza@redhat.com>