add "sudo ldconfig" when install liquid-dsp
add "sudo ldconfig" when install liquid-dsp

- add "sudo ldconfig" when install liquid-dsp

I install and execute demod command.
But it output error that not find liquid-dsp.so
I execute "sudo ldconfig" and can execute demod command.

ref.
https://www.raspberrypi.org/forums/viewtopic.php?t=150754

Thanks.
cubehub/demod