Vala Location demo can't find `libportal/portal.h`.

[rolandlo]

The Vala Location demo doesn't run on my system, since it can't find libportal/portal.h. I would assume that having the Vala extension installed should be enough to get all Vala demos running.

This was observed when porting the demo to Python in workbenchdev/demos#89

[sonnyp]

@Diego-Ivan wanna look into that?

[Diego-Ivan]

@sonnyp Sure, I'll take a look at it