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_crypto_server_handshaker.h b/quic/core/quic_crypto_server_handshaker.h
index b24e9e9..a67ea5e 100644
--- a/quic/core/quic_crypto_server_handshaker.h
+++ b/quic/core/quic_crypto_server_handshaker.h
@@ -91,7 +91,8 @@
  private:
   friend class test::QuicCryptoServerStreamPeer;
 
-  class ValidateCallback : public ValidateClientHelloResultCallback {
+  class QUIC_EXPORT_PRIVATE ValidateCallback
+      : public ValidateClientHelloResultCallback {
    public:
     explicit ValidateCallback(QuicCryptoServerHandshaker* parent);
     ValidateCallback(const ValidateCallback&) = delete;