#4 compilation issue for MingW

[wkliang]

the new added plc.[ch] work better than previous
version. the noise while opearting IVR system on
switching is gone.

iaxclient_lib.h (line 30) need a patch for MingW

#include <stdio.h>
#include <stdint.h> // add this one for int16_t
#include "spandsp/plc.h"

Thank you!

regards,
wkliang @ taipei.