Bluetooth: controller: split: openisa: fix missing include

If CONFIG_BT_CTLR_CONN_RSSI is enabled, then lll_conn.h should
be included as well. Otherwise, struct lll_conn is unknown
at the compile unit level. This has been reproduced by
compiling the hci_uart sample, where the following error occurs:

lll_adv.c: In function 'isr_rx_pdu':
lll_adv.c:722:13: error: dereferencing pointer to incomplete
                         type 'struct lll_conn'
    lll->conn->rssi_latest =  radio_rssi_get();
             ^~

Signed-off-by: Alex Porosanu <alexandru.porosanu@nxp.com>