[Kernel] Implement GetHeadphoneStatus, stub SignalEvent

src/core/kernel/kernel.cpp

@@ -25,6 +25,7 @@ void Kernel::serviceSVC(u32 svc) {
 		case 0x08: createThread(); break;
 		case 0x14: releaseMutex(); break;
 		case 0x17: createEvent(); break;
+		case 0x18: signalEvent(); break;
 		case 0x19: clearEvent(); break;
 		case 0x1F: mapMemoryBlock(); break;
 		case 0x21: createAddressArbiter(); break;