Add IWYU pragmas for deprecated headers

PiperOrigin-RevId: 490020560
diff --git a/upb/string_view.h b/upb/string_view.h
index 0e0fadd..add8275 100644
--- a/upb/string_view.h
+++ b/upb/string_view.h
@@ -26,6 +26,7 @@
  */
 
 // This header is deprecated, use upb/base/string_view.h instead
+// IWYU pragma: private, include "upb/base/string_view.h"
 
 #ifndef UPB_STRING_VIEW_H_
 #define UPB_STRING_VIEW_H_