sys/sysctl.h is required on OSX and probably other BSD

ext/libnatpmp/getgateway.c

@@ -53,6 +53,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #undef USE_PROC_NET_ROUTE
 #define USE_SOCKET_ROUTE
 #undef USE_SYSCTL_NET_ROUTE
+#include <sys/sysctl.h>
 #endif
 
 #ifdef __APPLE__

osdep/ManagedRoute.cpp

@@ -42,6 +42,7 @@
 #include <net/if.h>
 #ifdef __BSD__
 #include <net/if_dl.h>
+#include <sys/sysctl.h>
 #endif
 #include <ifaddrs.h>
 #endif

osdep/OSXEthernetTap.cpp

@@ -43,6 +43,7 @@
 #include <net/if_arp.h>
 #include <net/if_dl.h>
 #include <net/if_media.h>
+#include <sys/sysctl.h>
 #include <netinet6/in6_var.h>
 #include <netinet/in_var.h>
 #include <netinet/icmp6.h>