build: Drop redundant sys/sysctl.h header check

The `AC_CHECK_HEADERS` macro defines `HAVE_SYS_SYSCTL_H` if the
`sys/sysctl.h` header is found. However, in the source code, this header
is guarded by `HAVE_SYSCTL` and `HAVE_SYSCTL_ARND` macros, which have
their own checks. Since `HAVE_SYS_SYSCTL_H` is not used, we can skip the
`AC_CHECK_HEADERS(... sys/sysctl.h ...)` check.

Author: hebasto

configure.ac

@@ -896,7 +896,7 @@ if test "$TARGET_OS" = "darwin"; then
   AX_CHECK_LINK_FLAG([-Wl,-fixup_chains], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,-fixup_chains"], [], [$LDFLAG_WERROR])
 fi
 
-AC_CHECK_HEADERS([sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
+AC_CHECK_HEADERS([sys/select.h sys/prctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
 
 AC_CHECK_DECLS([getifaddrs, freeifaddrs],[CHECK_SOCKET],,
     [#include <sys/types.h>