From: Aman Karmani <aman@tmm1.net>

HAVE_SYSCTL is used to guard #include <sys/sysctl.h>, so make sure
we only define it when that header is present.

In recent glibc, the header was removed:
https://sourceware.org/pipermail/glibc-cvs/2020q2/069366.html

Signed-off-by: Aman Karmani <aman@tmm1.net>

Submitted-As: https://master.gitmailbox.com/ffmpegdev/pull.49.ffstaging.FFmpeg.1673910968885.ffmpegagent@gmail.com