Add QUIC_EXPORT_PRIVATE to prepare for presubmit

A subsequent CL (cl/276207410) will add a presubmit to mitigate QUICHE merge issues, this CL gets our code-base to a state of compliance with that presubmit. These haven't caused issues in Chromium if they were only used in the same build unit, but it's worth it to have a consistent use across the codebase to make sure everything works in Chromium.

gfe-relnote: no behavior change, QUIC_EXPORT_PRIVATE is a no-op in google3
PiperOrigin-RevId: 276276729
Change-Id: I67ab8c9df92d24d0006de7084ca563153f0bf07f
diff --git a/quic/core/quic_session.h b/quic/core/quic_session.h
index d221fe6..fba1e63 100644
--- a/quic/core/quic_session.h
+++ b/quic/core/quic_session.h
@@ -51,7 +51,7 @@
   // An interface from the session to the entity owning the session.
   // This lets the session notify its owner (the Dispatcher) when the connection
   // is closed, blocked, or added/removed from the time-wait list.
-  class Visitor {
+  class QUIC_EXPORT_PRIVATE Visitor {
    public:
     virtual ~Visitor() {}