Dibs on SAFETY_GM numerical value

To match Panda repo.

cereal/car.capnp

@@ -286,6 +286,7 @@ struct CarParams {
     honda @1;
     toyota @2;
     elm327 @3;
+    gm @4;
   }
 
   # things about the car in the manual

selfdrive/boardd/boardd.cc

@@ -35,6 +35,7 @@
 #define SAFETY_HONDA 1
 #define SAFETY_TOYOTA 2
 #define SAFETY_ELM327 0xE327
+#define SAFETY_GM 3
 
 namespace {
 
@@ -95,6 +96,9 @@ void *safety_setter_thread(void *s) {
   case (int)cereal::CarParams::SafetyModels::ELM327:
     safety_setting = SAFETY_ELM327;
     break;
+  case (int)cereal::CarParams::SafetyModels::GM:
+    safety_setting = SAFETY_GM;
+    break;
   default:
     LOGE("unknown safety model: %d", safety_model);
   }