Fix missing export specifier in HttpRequest. Closes #957.

Source/Urho3D/Network/HttpRequest.h

@@ -41,7 +41,7 @@ enum HttpRequestState
 };
 
 /// An HTTP connection with response data stream.
-class HttpRequest : public RefCounted, public Deserializer, public Thread
+class URHO3D_API HttpRequest : public RefCounted, public Deserializer, public Thread
 {
 public:
     /// Construct with parameters.