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

@@ -58,13 +58,16 @@
 #include <stdarg.h>
 #include <sys/time.h>
 #include <sys/types.h>
-#include <sys/sysctl.h>
 
 #include "xo_config.h"
 #include "xo.h"
 #include "xo_encoder.h"		/* For xo_realloc */
 #include "xo_buf.h"
 
+#ifdef HAVE_SYS_SYSCTL_H
+#include <sys/sysctl.h>
+#endif
+
 /*
  * SYSLOG (RFC 5424) requires an enterprise identifier.  This turns
  * out to be a fickle little issue.  For a single-vendor box, the