px4_main_t: fix method declaration throughout the code base

px4_main_t is defined as:
typedef int (*px4_main_t)(int argc, char *argv[]);
which matches with the definition in NuttX, given to task_create

src/modules/vtol_att_control/vtol_att_control_main.cpp

@@ -572,10 +572,11 @@ void VtolAttitudeControl::fill_fw_att_rates_sp()
 	}
 }
 
-void
+int
 VtolAttitudeControl::task_main_trampoline(int argc, char *argv[])
 {
 	VTOL_att_control::g_control->task_main();
+	return 0;
 }
 
 void VtolAttitudeControl::task_main()