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>

.qmake.conf

 load(qt_build_config)
-CONFIG += qt_example_installs
 CONFIG += warning_clean
 
 QT_SOURCE_TREE = $$PWD

mkspecs/features/qt_build_config.prf

@@ -71,6 +71,7 @@ CONFIG += \
     create_prl link_prl \
     prepare_docs qt_docs_targets \
     no_private_qt_headers_warning QTDIR_build \
+    qt_example_installs \
     # Qt modules get compiled without exceptions enabled by default.
     # However, testcases should be still built with exceptions.
     exceptions_off testcase_exceptions