utils: add libsss_child dependency to libsss_cert
    
    Since the refactoring of the ssh responder to call p11_child to
    validate certificates there is a dependency between libss_cert and
    libsss_child. In some environments, e.g. gentoo or the OpenSUSE build
    service, this dependency must be declared explicitly even if it is
    resolved otherwise while linking the binaries.
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>