Change QwtPlot Widget's <header> location in the *.ui file

Dear qwt-fans,

I've created an application for plotting graphs with qwt using Qt Creator.
I added the QwtPlot that I use to plot my graphs via drag and drop in the Qt Designer.

This creates automatically the following lines into my "mainwindow.ui":

<customwidget>
<class>QwtPlot</class>
<extends>QFrame</extends>
<header>qwt_plot.h</header>
<container>1</container>
</customwidget>

The problem is, that I want to use a different include path (qwt/qwt_plot.h). But the one in the <header> tag is hardcoded by the Designer, and is overwritten everytime I open my "mainwindow.ui" with the Designer.
Unfortunately, I can't find the setting how to override the include path for my qwtPlot.

What am I doing wrong and how can I fix this ? :)

Btw. My Qt version is 4.7.3 and qwt is 6.0.1.

Thanks in advance