QmlJS: Fix compiler warning and add a missing include

qmljsdiagnosticmessage_p.h: In static member function ‘static QmlJS::Severity::Enum QmlJS::DiagnosticMessage::qtMsgTypeToKind(QtMsgType)’:
qmljsdiagnosticmessage_p.h:72:5: warning: control reaches end of non-void function [-Wreturn-type]
   72 |     }
      |     ^

Change-Id: I2183199bf257453e0af2998b739fae8c90f5c098
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@qt.io>

src/libs/qmljs/parser/qmljsdiagnosticmessage_p.h

@@ -67,6 +67,7 @@ public:
         case QtFatalMsg:
             return Severity::Error;
         case QtInfoMsg:
+        default:
             return Severity::MaybeWarning;
         }
     }