Add QUIC_EXPORT_PRIVATE to ValueSplittingHeaderList::const_iterator.

Change-Id: Id0b0ab0b9b4789847c8d8d0fe92edf1201cde25d

quic/core/qpack/value_splitting_header_list.h

diff --git a/quic/core/qpack/value_splitting_header_list.h b/quic/core/qpack/value_splitting_header_list.h
index 2a8cdf0..d142e47 100644
--- a/quic/core/qpack/value_splitting_header_list.h
+++ b/quic/core/qpack/value_splitting_header_list.h
@@ -17,7 +17,7 @@
  public:
   using value_type = spdy::SpdyHeaderBlock::value_type;
 
-  class const_iterator {
+  class QUIC_EXPORT_PRIVATE const_iterator {
    public:
     // |header_list| must outlive this object.
     const_iterator(const spdy::SpdyHeaderBlock* header_list,