Bluetooth: hci_sync: Fix compilation warning

This fixes the following warning:

net/bluetooth/hci_sync.c:5143:5: warning: no previous prototype for
‘hci_le_ext_create_conn_sync’ [-Wmissing-prototypes]

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>