Fix standalone build.

Error: ./quiche/http2/adapter/http2_adapter.h:128:25: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
PiperOrigin-RevId: 700105509

quiche/http2/adapter/http2_adapter.h

diff --git a/quiche/http2/adapter/http2_adapter.h b/quiche/http2/adapter/http2_adapter.h
index 339bdf7..82a9171 100644
--- a/quiche/http2/adapter/http2_adapter.h
+++ b/quiche/http2/adapter/http2_adapter.h
@@ -125,7 +125,7 @@
                                 bool end_stream, void* user_data) = 0;
   // Deprecated
   int32_t SubmitRequest(absl::Span<const Header> headers,
-                        nullptr_t /*data_source*/, bool end_stream,
+                        std::nullptr_t /*data_source*/, bool end_stream,
                         void* user_data) {
     return SubmitRequest(headers, end_stream, user_data);
   }
@@ -138,7 +138,7 @@
                              bool end_stream) = 0;
   // Deprecated
   int SubmitResponse(Http2StreamId stream_id, absl::Span<const Header> headers,
-                     nullptr_t /*data_source*/, bool end_stream) {
+                     std::nullptr_t /*data_source*/, bool end_stream) {
     return SubmitResponse(stream_id, headers, end_stream);
   }