Fix CONFIG+=declarative_debug if declarative is implicitly added

Fix CONFIG+=declarative_debug if declarative is implicitly added

Change 1fb5600c5eb2ee2af8a766dea5b325aee6dd43b4 already added
declarative-debug as a way to include declarative, but nowadays
there's also qquick, and qquick-private ...

Just always set the define. The user has choosen to write
CONFIG+=declarative_debug in the first place and an additional
define if declarative isn't used does no harm.

Change-Id: Ica6142e70b12950fc1d9e0bf7b94f747c843a17f
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>