dbus: don't override EXTRA_OECONF

We need use EXTRA_OECONF_append_class-native rather than
EXTRA_OECONF_class-native.

(From OE-Core rev: 5e1560b98680c79c979b8f9325f9f9997f9fd438)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>