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

quic/core/quic_stream_sequencer_buffer.h

diff --git a/quic/core/quic_stream_sequencer_buffer.h b/quic/core/quic_stream_sequencer_buffer.h
index 93b723f..406cd92 100644
--- a/quic/core/quic_stream_sequencer_buffer.h
+++ b/quic/core/quic_stream_sequencer_buffer.h
@@ -85,7 +85,7 @@
   static const size_t kBlockSizeBytes = 8 * 1024;  // 8KB
 
   // The basic storage block used by this buffer.
-  struct BufferBlock {
+  struct QUIC_EXPORT_PRIVATE BufferBlock {
     char buffer[kBlockSizeBytes];
   };