AP_HAL_QURT: make in_main_thread const and override

libraries/AP_HAL_QURT/Scheduler.cpp

@@ -261,7 +261,7 @@ void *Scheduler::_io_thread(void *arg)
     return nullptr;
 }
 
-bool Scheduler::in_main_thread()
+bool Scheduler::in_main_thread() const
 {
     return getpid() == _main_task_pid;
 }

libraries/AP_HAL_QURT/Scheduler.h

@@ -31,7 +31,7 @@ public:
     void     resume_timer_procs();
     void     reboot(bool hold_in_bootloader);
 
-    bool     in_main_thread();
+    bool     in_main_thread() const override;
     void     system_initialized();
     void     hal_initialized();