Avoid tripping -Wchanges-meaning in GCC

I got this warning building QUICHE with GCC because I forgot to `CC=clang`:

```
quiche/quic/core/qpack/qpack_encoder_stream_sender_test.cc:28:19: error: declaration of 'quic::HuffmanEncoding quic::test::{anonymous}::QpackEncoderStreamSenderTest::HuffmanEncoding()' changes meaning of 'HuffmanEncoding' [-Wchanges-meaning]
   28 |   HuffmanEncoding HuffmanEncoding() {
      |                   ^~~~~~~~~~~~~~~
quiche/quic/core/qpack/qpack_encoder_stream_sender_test.cc:28:3: note: used here to mean 'enum class quic::HuffmanEncoding'
   28 |   HuffmanEncoding HuffmanEncoding() {
      |   ^~~~~~~~~~~~~~~
```

The fix is easy enough.

PiperOrigin-RevId: 698519906
diff --git a/quiche/quic/core/qpack/qpack_encoder_stream_sender_test.cc b/quiche/quic/core/qpack/qpack_encoder_stream_sender_test.cc
index 823cae8..3ebc8f6 100644
--- a/quiche/quic/core/qpack/qpack_encoder_stream_sender_test.cc
+++ b/quiche/quic/core/qpack/qpack_encoder_stream_sender_test.cc
@@ -19,13 +19,13 @@
 
 class QpackEncoderStreamSenderTest : public QuicTestWithParam<bool> {
  protected:
-  QpackEncoderStreamSenderTest() : stream_(HuffmanEncoding()) {
+  QpackEncoderStreamSenderTest() : stream_(GetHuffmanEncoding()) {
     stream_.set_qpack_stream_sender_delegate(&delegate_);
   }
   ~QpackEncoderStreamSenderTest() override = default;
 
   bool DisableHuffmanEncoding() { return GetParam(); }
-  HuffmanEncoding HuffmanEncoding() {
+  HuffmanEncoding GetHuffmanEncoding() {
     return DisableHuffmanEncoding() ? HuffmanEncoding::kDisabled
                                     : HuffmanEncoding::kEnabled;
   }