samples: net: mdns_responder: Increase ZVFS_POLL_MAX

The sample needs a bit more resources. The existing build
assert complains about this

error: static assertion failed: "CONFIG_ZVFS_POLL_MAX must be larger than
 ((1 + (2 + 0)) + (3 + 3) + 0)"
include/zephyr/net/dns_resolve.h:146:1: note: in expansion of macro
 'BUILD_ASSERT'
  146 | BUILD_ASSERT(CONFIG_ZVFS_POLL_MAX >= DNS_DISPATCHER_MAX_POLL,
      | ^~~~~~~~~~~~

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>