Linux: Deprecate <sys/sysctl.h> and sysctl

Now that there are no internal users of __sysctl left, it is possible
to add an unconditional deprecation warning to <sys/sysctl.h>.

To avoid a test failure due this warning in check-install-headers,
skip the test for sys/sysctl.h.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>