ipa-sidgen: make internal fetch_attr helper really internal
    
    With 389-ds landing a change for
    https://pagure.io/389-ds-base/issue/49950, fetch_attr() helper function
    is exposed in slapi-plugin.h. However, in order to be able to build
    FreeIPA plugins against older 389-ds versions, prefer using a local
    variant of it.
    
    Rename fetch_attr() to ipa_sidgen_fetch_attr() so that it doesn't
    conflict at all.
    
    Fixes: https://pagure.io/freeipa/issue/7811
    Reviewed-By: Christian Heimes <cheimes@redhat.com>