libobs: add missing FreeBSD #include to fix build

sysctlbyname requires #include <sys/sysctl.h>.  Perhaps this previously
worked due to header pollution that has since been cleaned up in newer
FreeBSD.

libobs/util/platform-nix.c

@@ -38,6 +38,7 @@
 #include <sys/param.h>
 #include <sys/queue.h>
 #include <sys/socket.h>
+#include <sys/sysctl.h>
 #include <sys/user.h>
 #include <unistd.h>
 #include <libprocstat.h>