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.

Signed-off-by: Marcus Asteborg <maastebo@microsoft.com>
Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>
4 jobs for arm-rtcd in 4 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #10377
docker
autoconf

00:04:44

passed #10378
docker
cmake

00:02:14

 
  External
passed continuous-integration/appveyor/branch

passed continuous-integration/appveyor/merge-request