add platform definition for IA64

[matoro]

How I tested: * built ffmpeg with libilbc enabled * obtained a sample file from https://web.archive.org/web/2016*/http://www.andrews-corner.org/samples/luckynight.lbc * converted it to wav using ffmpeg on ia64 host * compared to file converted on amd64 host * did the same in reverse (converted wav to lbc)

All comparisons were identical and sounded the same.