OpenVG plugin doesn't build with the Mesa implementation of OpenVG

The problem is that Mesa's implementation of OpenVG drags in a couple of X11 headers that do things that clash with Qt headers, such as

#define None 0
#define Bool int

etc.

I'm attaching a patch that "fixes" it the quick and dirty way, but it's so ugly that someone should probably go over it and fix it properly (that's why I'm not sending a merge request).