spec: add hostname.rpm into Recommends list
    
    kexec-tools runs hostname binary in the case of fence_kdump. Since this
    is a trival dependency and should not block the kexec-tools installation
    if non-existent, using weak-dependency to resolve it.
    
    Signed-off-by: Pingfan Liu <piliu@redhat.com>
    Acked-by: Tao Liu <ltao@redhat.com>