Codechange: Specify underlying type for all enums excluding those exposed to scripts. (#13383)

src/signal_type.h

@@ -39,7 +39,7 @@ DECLARE_ENUM_AS_ADDABLE(SignalType)
  * simple boolean logic will do. But do try to compare to this enum instead of
  * normal boolean evaluation, since that will make future additions easier.
  */
-enum SignalState {
+enum SignalState : uint8_t {
 	SIGNAL_STATE_RED   = 0, ///< The signal is red
 	SIGNAL_STATE_GREEN = 1, ///< The signal is green
 };