Put back OTHER_FILES in systemdialogs example

Partial revert of 1e9589eb0c2bb9404e11c69e1d76ef9efd89af56.
Creator currently needs this variable in order to find the QML files.
(see https://bugreports.qt-project.org/browse/QTCREATORBUG-9860)

Change-Id: Id5882dfe9733a6b401976e4d6e5baf2066d21b1b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>