logger: move pthread_t cast to pid_t into watchdog_initialize

avoids compile error on OSX, trying to cast pthread_t to pid_t

src/modules/logger/logger.cpp

@@ -960,12 +960,11 @@ void Logger::run()
 		if (_writer.backend() & LogWriter::BackendFile) {
 
 			const pid_t pid_self = getpid();
-			// The pthread_t ID is equal to the PID on NuttX
-			const pid_t pid_writer = _writer.thread_id_file();
+			const pthread_t writer_thread = _writer.thread_id_file();
 
 			// sched_note_start is already called from pthread_create and task_create,
 			// which means we can expect to find the tasks in system_load.tasks, as required in watchdog_initialize
-			watchdog_initialize(pid_self, pid_writer, timer_callback_data.watchdog_data);
+			watchdog_initialize(pid_self, writer_thread, timer_callback_data.watchdog_data);
 		}
 
 		hrt_call_every(&timer_call, _log_interval, _log_interval, timer_callback, &timer_callback_data);

src/modules/logger/watchdog.cpp

@@ -133,10 +133,13 @@ bool watchdog_update(watchdog_data_t &watchdog_data)
 
 }
 
-void watchdog_initialize(const pid_t pid_logger_main, const pid_t pid_logger_writer, watchdog_data_t &watchdog_data)
+void watchdog_initialize(const pid_t pid_logger_main, const pthread_t writer_thread, watchdog_data_t &watchdog_data)
 {
 #ifdef __PX4_NUTTX
 
+	// The pthread_t ID is equal to the PID on NuttX
+	const pthread_t pid_logger_writer = writer_thread;
+
 	sched_lock(); // need to lock the tcb access
 
 	for (int i = 0; i < CONFIG_MAX_TASKS; i++) {

src/modules/logger/watchdog.h

@@ -58,7 +58,7 @@ struct watchdog_data_t {
 /**
  * Initialize the watchdog, fill in watchdog_data.
  */
-void watchdog_initialize(const pid_t pid_logger_main, const pid_t pid_logger_writer, watchdog_data_t &watchdog_data);
+void watchdog_initialize(const pid_t pid_logger_main, const pthread_t writer_thread, watchdog_data_t &watchdog_data);
 
 /**
  * Update the watchdog and trigger it if necessary. It is triggered when the log writer task is in