graphene: remove introspection from PACKAGECONFIG

Assignment of "introspection" as default PACKAGECONFIG value does not
reflect gobject-introspection being globally enabled or disabled.
As a result, graphene compilation used to fail when g-i was disabled.

Use standard means to enable or disable introspection based on machine
and distro features, as implemented by gobject-introspection class.

Signed-off-by: Petr Kubizňák <kubiznak@2n.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>