Add HOST_PROMPT_SUPPORT (#13039)

Marlin/src/feature/emergency_parser.cpp

@@ -34,6 +34,10 @@
 bool EmergencyParser::killed_by_M112, // = false
      EmergencyParser::enabled;
 
+#if ENABLED(HOST_PROMPT_SUPPORT)
+  uint8_t EmergencyParser::M876_reason; // = 0
+#endif
+
 // Global instance
 EmergencyParser emergency_parser;