illumos lacks support for Type of Service in ip.h
We need to add missing defines

--- ngtcp2-1.15.1/examples/shared.cc.orig	2025-08-31 10:32:32.431072168 +0200
+++ ngtcp2-1.15.1/examples/shared.cc	2025-09-07 18:30:00.110096007 +0200
@@ -54,6 +54,11 @@
 
 namespace ngtcp2 {
 
+#ifdef __illumos__
+    #define	IPTOS_ECN_MASK		0x03
+    #define	IPTOS_ECN(x)		((x) & IPTOS_ECN_MASK)
+#endif
+
 uint8_t msghdr_get_ecn(msghdr *msg, int family) {
   switch (family) {
   case AF_INET: