Remove sys/platform/ppc.h include. (#678)
Remove sys/platform/ppc.h include. (#678)

According to the comment this include was used for
`__ppc_get_timebase_freq` at some point in this file but nowadays that
function is not there, so we can remove the include. Fixes #677.
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl