Add QUICHE_EXPORT annotation to class quic::MetadataDecoder::MetadataHeadersDecoder

`quic::MetadataDecoder::MetadataHeadersDecoder` is used in tests but didn't have a `QUICHE_EXPORT` annotation. This causes linking errors in Chromium on Windows. On non-Windows platforms the `QUICHE_EXPORT` annotation from the nesting class applies, but that doesn't work on Windows.

Add the annotation.

No functional change.

PiperOrigin-RevId: 1018799782

quiche/quic/core/http/metadata_decoder.h

diff --git a/quiche/quic/core/http/metadata_decoder.h b/quiche/quic/core/http/metadata_decoder.h
index c17bfca..55b2911 100644
--- a/quiche/quic/core/http/metadata_decoder.h
+++ b/quiche/quic/core/http/metadata_decoder.h
@@ -39,7 +39,7 @@
   const QuicHeaderList& headers() { return decoder_.headers(); }
 
  private:
-  class MetadataHeadersDecoder
+  class QUICHE_EXPORT MetadataHeadersDecoder
       : public QpackDecodedHeadersAccumulator::Visitor {
    public:
     // QpackDecodedHeadersAccumulator::Visitor