guard sysctl.h include with pre-existing autoconf check

* HAVE_SYSCTLBYNAME and HAVE_SYS_SYSCTL could be paired together as
  sysctlbyname isn't available if sysctl.h isn't available.
* Sysctl syscall isn't available in linux >=5.5 and the header
  is absent in glibc-2.32 and musl-1.0.0

Closes: #87
Signed-off-by: Alfred Wingate <parona@protonmail.com>

libxo/xo_syslog.c

@@ -58,7 +58,9 @@
 #include <stdarg.h>
 #include <sys/time.h>
 #include <sys/types.h>
+#ifdef HAVE_SYS_SYSCTL
 #include <sys/sysctl.h>
+#endif
 
 #include "xo_config.h"
 #include "xo.h"