Add a missing IWYU pragma for third_party/http2/adapter/nghttp2.h.

PiperOrigin-RevId: 560759018
diff --git a/quiche/http2/adapter/nghttp2.h b/quiche/http2/adapter/nghttp2.h
index eed3c86..e19094e 100644
--- a/quiche/http2/adapter/nghttp2.h
+++ b/quiche/http2/adapter/nghttp2.h
@@ -6,6 +6,6 @@
 // Required to build on Windows.
 using ssize_t = ptrdiff_t;
 
-#include "nghttp2/nghttp2.h"
+#include "nghttp2/nghttp2.h"  // IWYU pragma: export
 
 #endif  // QUICHE_HTTP2_ADAPTER_NGHTTP2_H_