add unreachable statement

Author: ludviggunne

lib/errorlogger.cpp

@@ -971,6 +971,7 @@ std::string getClassification(const std::string &guideline, ReportType reportTyp
             info = isDirective ? &checkers::misraC2025Directives : &checkers::misraC2025Rules;
             break;
         default:
+            cppcheck::unreachable();
             break;
         }