gobject-introspection: avoid using += with an over-ride
Using += with an over-ride can be a source of confusion so try to
avoid the construct in core recipes.
The current usage functions correctly (it over-rides the default, ie
empty, value of EXTRA_OECONF and then PACKAGECONFIG derived options
are subsequently appended), however the += is unnecessary and can be
dropped.
Signed-off-by: Andre McCurdy <armccurdy@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
