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>

sysdeps/unix/sysv/linux/sysctl.c

@@ -17,8 +17,7 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <errno.h>
-#include <string.h>	/* For the real memset prototype.  */
-#include <sys/sysctl.h>
+#include <linux/sysctl.h>
 
 #include <sysdep.h>
 #include <sys/syscall.h>
@@ -39,5 +38,4 @@ __sysctl (int *name, int nlen, void *oldval, size_t *oldlenp,
 
   return INLINE_SYSCALL (_sysctl, 1, &args);
 }
-libc_hidden_def (__sysctl)
 weak_alias (__sysctl, sysctl)