Add missing include to quic_types.h.

This unblocks removing unused includes from Chromium's quic_flags_impl.h.

PiperOrigin-RevId: 425952785

quic/core/quic_types.h

diff --git a/quic/core/quic_types.h b/quic/core/quic_types.h
index 6960316..a437c9a 100644
--- a/quic/core/quic_types.h
+++ b/quic/core/quic_types.h
@@ -14,6 +14,7 @@
 
 #include "absl/container/inlined_vector.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
 #include "quic/core/quic_connection_id.h"
 #include "quic/core/quic_error_codes.h"
 #include "quic/core/quic_packet_number.h"