gdbus-codegen: Fix issue with docbook generation
gdbus-codegen: Fix issue with docbook generation

In cases where gdbus-codegen is used only for docbook generation,
the execution stops with the following error message:

  `Using --header or --body requires --output`

This is because it was assumed that, in addition to the docbook
generation, the header or source code were always generated.

This patch fixes this, and the header or source code generation
is not mandatory, so the docbook can be generated separately.

https://bugzilla.gnome.org/show_bug.cgi?id=791015

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib