enable example installs for all modules

so far, each module had to do it in its .qmake.conf. by now, all modules
have been adjusted to the expected structure, so we can enable it
centrally.

Change-Id: I16b4e7581e51bfc54e71d2f7f852858ae0b47281
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>