QmakeProject: Use QMAKE_EXT_H to generate correct ui_h paths

So that the code model can find the right files for ui files
that haven't been build.

Change-Id: I438a3ab9806c8ff6c47e5e188228a31ffb917bdf
Task-number: QTCREATORBUG-14910
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -899,7 +899,7 @@ QString QmakeProject::generatedUiHeader(const FileName &formFile) const
         if (const QmakeProFileNode *pro = proFileNodeOf(m_rootProjectNode, FormType, formFile))
             return QmakeProFileNode::uiHeaderFile(
                         pro->uiDirectory(Utils::FileName::fromString(pro->buildDir())),
-                        formFile);
+                        formFile, pro->singleVariableValue(QmakeVariable::UiHeaderExtensionVar));
     return QString();
 }