Add QUICHE_EXPORT to quiche::ObliviousHttpRequest::Context

This is causing a linking error on Windows for the OHTTP encryption CL.

PiperOrigin-RevId: 485645363

quiche/oblivious_http/buffers/oblivious_http_request.h

diff --git a/quiche/oblivious_http/buffers/oblivious_http_request.h b/quiche/oblivious_http/buffers/oblivious_http_request.h
index 5ff195c..58a555b 100644
--- a/quiche/oblivious_http/buffers/oblivious_http_request.h
+++ b/quiche/oblivious_http/buffers/oblivious_http_request.h
@@ -21,7 +21,7 @@
   // Holds the HPKE related data received from request. This context is created
   // during request processing, and subsequently passed into response handling
   // in `ObliviousHttpResponse`.
-  class Context {
+  class QUICHE_EXPORT Context {
    public:
     ~Context() = default;