[style] declare exported headers (#12255)

This commit adds IWYU pragma to declare exported header files, so that
IWYU won't complain about our current include style.

include/openthread/config.h

@@ -41,7 +41,7 @@
  * The OpenThread feature configuration file.
  */
 #if defined(OPENTHREAD_CONFIG_FILE)
-#include OPENTHREAD_CONFIG_FILE
+#include OPENTHREAD_CONFIG_FILE // IWYU pragma: export
 #endif
 
 #endif // OPENTHREAD_CONFIG_H_

include/openthread/instance.h

@@ -52,7 +52,7 @@ extern "C" {
  *
  * @note This number versions both OpenThread platform and user APIs.
  */
-#define OPENTHREAD_API_VERSION (565)
+#define OPENTHREAD_API_VERSION (566)
 
 /**
  * @addtogroup api-instance

src/core/openthread-core-config.h

@@ -34,7 +34,7 @@
 #ifndef OT_CORE_OPENTHREAD_CORE_CONFIG_H_
 #define OT_CORE_OPENTHREAD_CORE_CONFIG_H_
 
-#include <openthread/config.h>
+#include <openthread/config.h> // IWYU pragma: export
 #include <openthread/thread.h>
 
 #define OPENTHREAD_CORE_CONFIG_H_IN
@@ -48,21 +48,22 @@
  */
 
 #ifdef OPENTHREAD_PROJECT_CORE_CONFIG_FILE
-#include OPENTHREAD_PROJECT_CORE_CONFIG_FILE
+#include OPENTHREAD_PROJECT_CORE_CONFIG_FILE // IWYU pragma: export
 #elif defined(OPENTHREAD_CONFIG_CORE_USER_CONFIG_HEADER_ENABLE)
 // This configuration header file should be provided by the user when
 // OPENTHREAD_CONFIG_CORE_USER_CONFIG_HEADER_ENABLE is defined to 1.
-#include "openthread-core-user-config.h"
+#include "openthread-core-user-config.h" // IWYU pragma: export
 #endif
 
 #ifdef OPENTHREAD_PLATFORM_CORE_CONFIG_FILE
-#include OPENTHREAD_PLATFORM_CORE_CONFIG_FILE
+#include OPENTHREAD_PLATFORM_CORE_CONFIG_FILE // IWYU pragma: export
 #endif
 
 #ifndef OPENTHREAD_CONFIG_THREAD_VERSION
 #define OPENTHREAD_CONFIG_THREAD_VERSION OT_THREAD_VERSION_1_3
 #endif
 
+// IWYU pragma: begin_exports
 #include "config/announce_sender.h"
 #include "config/backbone_router.h"
 #include "config/border_agent.h"
@@ -111,6 +112,7 @@
 #include "config/tmf.h"
 #include "config/trel.h"
 #include "config/wakeup.h"
+// IWYU pragma: end_exports
 
 #undef OPENTHREAD_CORE_CONFIG_H_IN