QmlJS: Add error message for Component with multiple children

Component is only allowed to have a single child element, that will
be the root element of the component.

If there is no child at all we create a warning. Having no child is
temporarily required.

Change-Id: I5c0d9d9cdf1be106b20ed4f1134a973d58126498
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/libs/qmljs/qmljscheck.h

@@ -158,6 +158,7 @@ private:
 
     bool _importsOk;
     bool _inStatementBinding;
+    int _componentChildCount = 0;
     const Imports *_imports;
     TranslationFunction lastTransLationfunction = noTranslationfunction;
 };