Fix arm build with rtcd enabled.
The autotools build doesn't set OPUS_HAVE_RTCD for arm targets, assuming all the supported intrinsics will work on the runtime cpu.
The cmake build however defines this by default when the neon extension is available on the target. On Linux, the runtime cpu detection reads /proc/cpuinfo, so removing stdio.h from celt/os_support.h meant that the cmake build for arm targets failed.
We don't currently have ci runs for that configuration, so this only became apparent through manual testing.