AP_PeriodicProcessStub: fixed build due to missing reference to _suspended

libraries/AP_PeriodicProcess/AP_PeriodicProcessStub.cpp

@@ -16,7 +16,7 @@ void AP_PeriodicProcessStub::suspend_timer(void) {
 void AP_PeriodicProcessStub::resume_timer(void) {
 }
 bool AP_PeriodicProcessStub::running(void) {
-    return !_suspended;
+    return false;
 }
 void AP_PeriodicProcessStub::run(void) {
 }