Update libiscsi to rev 13 via SR 808956
    
    https://build.opensuse.org/request/show/808956
    by user pluskalm + maxlin_factory
    - Update to version 1.19.0+git.20200521:
      * test-tool, compare and write: Reduce maximum number of blocks from 256 to 255
      * test_compareandwrite_invalid_dataout_size: Simplify this test
      * test-tool: Use asprintf() in sg_send_scsi_cmd()
      * test-tool: Fix a comment in sg_send_scsi_cmd()
      * test-tool: Split send_scsi_command()
      * socket: fix rewrite cmdsn of immediate pdus
      * iser: eliminate unnecessary memory allocations
      * iser: use `login_resp_buf` until login is finished
      * init: free iscsi->opaque before check mallocs/frees counter
      * reconnect: collect mallocs/frees of the previous reconnection