ekf2: raise stack size on Snapdragon

It seems that the stack size was exhausted on Snapdragon.

src/modules/ekf2/ekf2_main.cpp

@@ -873,11 +873,19 @@ int Ekf2::start()
 {
 	ASSERT(_control_task == -1);
 
+#ifdef __PX4_QURT
+	// On the DSP we seem to get random crashes with a stack size below 13000.
+	const unsigned stack_size = 15000;
+#else
+	const unsigned stack_size = 9000;
+#endif
+
+
 	/* start the task */
 	_control_task = px4_task_spawn_cmd("ekf2",
 					   SCHED_DEFAULT,
 					   SCHED_PRIORITY_MAX - 5,
-					   9000,
+					   stack_size,
 					   (px4_main_t)&Ekf2::task_main_trampoline,
 					   nullptr);