Update libpipeline to version 1.5.6 / rev 14 via SR 974422
    
    https://build.opensuse.org/request/show/974422
    by user WernerFink + dimstar_suse
    - update to 1.5.6:
      * Fix handling of leading whitespace in `pipecmd_new_argstr` and
       `pipecmd_argstr`. (forwarded request 973755 from dirkmueller)