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>
744e829637