XML writer: resolve declaration-only enum definitions

XML writer: resolve declaration-only enum definitions

Let the writer look through declaration-only enums for definitions.

This matches what get_preferred_type, write_class_decl and
write_union_decl do. No current test cases are affected.

	* src/abg-writer.cc (write_enum_type_decl): Look through
	declaration-only types the same as for structs and unions.

Reviewed-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Matthias Maennich <maennich@google.com>