Replace %{_libdir} macro in BuildRequires
    
    The %{_libdir} macro is architecture dependend and therefore does not
    correctly work across different platforms. In the past the SRPM was
    created on a platform with /usr/lib64. Recent SRPMs have /usr/lib, which
    breaks dnf builddep.
    
    Depend on krb5-server directly rather than a file in krb5-server
    package:
    
    $ rpm -qf /usr/lib64/krb5/plugins/kdb/db2.so
    krb5-server-1.16.1-25.fc29.x86_64
    
    Fixes: https://pagure.io/freeipa/issue/8056
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Robbie Harwood <rharwood@redhat.com>
    Reviewed-By: François Cami <fcami@redhat.com>