People expect that building the "main target" of an example builds all
necessary subtargets as well. Add a dependency from plugandpaint to
pnp_extrafilters.
Pick-to: 6.5
Task-number: QTBUG-112300
Change-Id: I036beb961fe474ff060f93c98f5e2bda58e66f71
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>