tools/build: Provide modern sysctl.h for bootstrap tools

[markj]

Summary

When building from 13, bootstrapping certctl fails because certctl.c
includes sys/sysctl.h but not sys/types.h. Prior to commit
56ee5c551f89e consumers were required to include sys/types.h explicitly
to provide a definition for size_t.

Since the user-facing portion of sys/sysctl.h hasn't changed otherwise,
just add it to SYSINCS on FreeBSD to work around this problem. We
already provide a sys/sysctl.h for cross-building from other OSs.

[markj]

Any comments on this patch? Am I going about this the wrong way?

[jrtc27]

Just because sys/sysctl.h is self-contained doesn't mean certctl.c shouldn't also be including sys/types.h. Presumably it itself is using types from that header and so should explicitly include it, regardless of whether sys/sysctl.h is including it for its own needs (which is an implementation detail; what means it uses to give things it defines the right types is up to it). And given style(9) header ordering rules that would mean including sys/types.h first, which would render this obsolete.