Add missing return in Set_Serial_Settings

Another one of those "warnings are good, use them" moments.  :-)

src/os/win32/dev-serial.c

@@ -108,6 +108,7 @@ static REBINT Set_Serial_Settings(HANDLE h, REBREQ *req)
 	}
 
 	PurgeComm(h,PURGE_RXCLEAR|PURGE_TXCLEAR);  //make sure buffers are clean
+    return 0;
 }
 
 /***********************************************************************