Build demo examples directory

The directory will host more demos in the future so it is
wise to enable CI to build it and report issues, if any.

Change-Id: I97fbe8fc7589ec2c18e1dcb32c02a40332f9453b
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>