Fix compilation error under GCC.

Error message:
error: attribute ignored [-Werror=attributes]
  427 | #define ABSL_MUST_USE_RESULT [[nodiscard]]
      |                              ^
bazel-out/k8-fastbuild/bin/external/com_googlesource_quiche/quiche/quic/core/web_transport_interface.h:49:11: note: in expansion of macro 'ABSL_MUST_USE_RESULT'
   49 |   virtual ABSL_MUST_USE_RESULT ReadResult Read(char* buffer,
PiperOrigin-RevId: 367301659
Change-Id: I9d07e5c8dee0a5f1cc9c4f159d3631f32d674456

quic/core/web_transport_interface.h

diff --git a/quic/core/web_transport_interface.h b/quic/core/web_transport_interface.h
index 0755d9e..154d242 100644
--- a/quic/core/web_transport_interface.h
+++ b/quic/core/web_transport_interface.h
@@ -46,14 +46,14 @@
   virtual ~WebTransportStream() {}
 
   // Reads at most |buffer_size| bytes into |buffer|.
-  virtual ABSL_MUST_USE_RESULT ReadResult Read(char* buffer,
+  ABSL_MUST_USE_RESULT virtual ReadResult Read(char* buffer,
                                                size_t buffer_size) = 0;
   // Reads all available data and appends it to the end of |output|.
-  virtual ABSL_MUST_USE_RESULT ReadResult Read(std::string* output) = 0;
+  ABSL_MUST_USE_RESULT virtual ReadResult Read(std::string* output) = 0;
   // Writes |data| into the stream.  Returns true on success.
-  virtual ABSL_MUST_USE_RESULT bool Write(absl::string_view data) = 0;
+  ABSL_MUST_USE_RESULT virtual bool Write(absl::string_view data) = 0;
   // Sends the FIN on the stream.  Returns true on success.
-  virtual ABSL_MUST_USE_RESULT bool SendFin() = 0;
+  ABSL_MUST_USE_RESULT virtual bool SendFin() = 0;
 
   // Indicates whether it is possible to write into stream right now.
   virtual bool CanWrite() const = 0;