Add IWYU pragmas for deprecated headers

PiperOrigin-RevId: 490020560
diff --git a/upb/encode.h b/upb/encode.h
index 14ce190..37d424f 100644
--- a/upb/encode.h
+++ b/upb/encode.h
@@ -26,6 +26,7 @@
  */
 
 // This header is deprecated, use upb/wire/encode.h instead
+// IWYU pragma: private, include "upb/wire/encode.h"
 
 #ifndef UPB_ENCODE_H_
 #define UPB_ENCODE_H_