net: samples: Enable CONFIG_NET_CONTEXT_NBUF_POOL in prj_bt.conf

Bluetooth consumes buffer when transmitting them therefore any protocol
that need to retain them, like TCP, needs to have its own pool to copy
the buffers as they may need to be resent in case they are not acked.

Change-Id: I483c5a3eb4d54fae6b5b2902c69b67e8c2e0be5e
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>