Build the photosurface demo without Qt PDF

The photosurface demo is only built when Qt PDF is available, because
its intro slides are the pages of a PDF file, shown through Qt PDF's
image format plugin. The demo doesn't link to Qt PDF, though, so it
builds without it. When the plugin is missing, the intro image fails to
load and the demo goes straight to the photo surface.

This change ensures the demo is built whenever Qt Quick and qsb are
available, so CI keeps compiling it on configurations also without
Qt PDF.

Task-number: QTBUG-151373
Change-Id: Ic80657b4143b9d74b6e269baea6fbb9d051f1f73
Reviewed-by: Topi Reinio <topi.reinio@qt.io>