Deprecated use of <sys/sysctl.h>

I've been getting warnings since couple days ago:

/usr/include/x86_64-linux-gnu/sys/sysctl.h:21:2: warning: #warning "The <sys/sysctl.h> header is deprecated and will be removed." [-Wcpp]
   21 | #warning "The <sys/sysctl.h> header is deprecated and will be removed."

It is annoying with enable-gcc-warnings since it errors.

I've removed that include and tor builds fine so, as nickm suggested on IRC, right solution here is to not include on Linux/Win32 instead of #ifdef HAVE_SYS_SYSCTL_H.

I would recommend a backport since if this becomes a problem to build tor in the future.

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