llext: add explicit cast to fix Coverity CID: 392507

From https://mails.dpdk.org/archives/dev/2021-December/231212.html

:

    Downcasting a void* to struct aesni_gcm_session* caused the session
    data to be treated as tainted. Removing the void* temporary variable
    and adding a cast avoids this issue.

Try the same approach here to prevent the ldr->sect_hdrs pointer from
being treated as tainted.

May fix #74817.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>