GDBusObjectSkeleton objects are larger than GObject, so we need to
allocate enough space. Otherwise, if we're lucky we'll get:

    Sep 13 11:09:32 espresso gsd-smartcard[12882]: specified class size for type 'GsdSmartcardService' is smaller than the parent type's 'GsdSmartcardServiceManagerSkeleton' class size
    Sep 13 11:09:32 espresso gsd-smartcard[12882]: g_type_add_interface_static: assertion 'G_TYPE_IS_INSTANTIATABLE (instance_type)' failed

and if we're unlucky we'll get object fields overwriting each other.

Signed-off-by: Simon McVittie <smcv@debian.org>
Fixes: 76b6b794 "smartcard: Use G_DECLARE_FINAL_TYPE for GsdSmartcardService"