Bluetooth: Set link key generation bit if necessary for LE SC

Depending on whether Secure Connections is enabled or not we may need to add
the link key generation bit to the key distribution. This patch does the
necessary modifications to the build_pairing_cmd() function.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>