parser-inc: Add stub header for sys/sysctl.h for BSD (incl. macOS)

dtool/src/parser-inc/sys/sysctl.h

@@ -0,0 +1,15 @@
+/**
+ * Stub header for sys/sysctl.h in macOS and FreeBSD.
+ */
+
+#pragma once
+
+struct ctlname {
+  char *ctl_name;
+  int ctl_type;
+};
+
+struct sysctl_req;
+struct sysctl_oid;
+struct sysctl_ctx_entry;
+struct sysctl_ctx_list;