network: avoid including sys/sysctl.h on Linux

network: avoid including sys/sysctl.h on Linux

The sys/sysctl.h header is only needed on BSD platforms to get
the sysctlbyname() function declaration. On Linux we talk to
procfs instead to change sysctls.

Unfortunately a legacy sys/sysctl.h header does exist on Linux
and including it has recently started triggering a deprecation
warning from glibc.

Protect its inclusion with a HAVE_SYSCTLBYNAME check instead
so that it only gets used on platforms where we need that
function declaration.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>