Do not use <sys/sysctl.h> on Linux

glibc 2.30 deprecated the <sys/sysctl.h> header [1].  However, that
header is still used on other Unix-like systems, so only prevent it from
being used on Linux, in order to prevent compiler warnings from being
triggered.

[1] https://sourceware.org/ml/libc-alpha/2019-08/msg00029.html

lib/isc/unix/meminfo.c

@@ -12,7 +12,7 @@
 #include <isc/meminfo.h>
 #include <inttypes.h>
 #include <unistd.h>
-#ifdef HAVE_SYS_SYSCTL_H
+#if defined(HAVE_SYS_SYSCTL_H) && !defined(__linux__)
 #include <sys/sysctl.h>
 #endif
 

lib/isc/unix/net.c

@@ -12,7 +12,7 @@
 #include <stdbool.h>
 #include <sys/types.h>
 
-#if defined(HAVE_SYS_SYSCTL_H)
+#if defined(HAVE_SYS_SYSCTL_H) && !defined(__linux__)
 #if defined(HAVE_SYS_PARAM_H)
 #include <sys/param.h>
 #endif

lib/isc/unix/socket.c

@@ -18,7 +18,7 @@
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
-#ifdef HAVE_SYS_SYSCTL_H
+#if defined(HAVE_SYS_SYSCTL_H) && !defined(__linux__)
 #include <sys/sysctl.h>
 #endif
 #include <sys/time.h>