codegen: Fix use of uninitialised variable

If using the --interface-info-{body,header} options to gdbus-codegen, and the first interface to be outputted has no methods, but does have properties or signals, an uninitialised variable would be used for the property/signal ‘since’ values.

In other situations, the ‘since’ value for a prior method would have been incorrectly used for the properties/signals.

Signed-off-by: Philip Withnall withnall@endlessm.com