abidiff,reader: Fix compilation with --debug-self-comparison

abidiff,reader: Fix compilation with --debug-self-comparison

While chasing self-comparison changes errors, I realized that
compilation is failing when the package is configured with
--debug-self-comparison.

Fixed thus.

	* src/abg-reader.cc
	(reader::{maybe_check_abixml_canonical_type_stability, read_corpus}):
	get_environment() doesn't returns a pointer anymore.  So adapt the
	code.
	(maybe_map_type_with_type_id): Adjust because the environment is
	const.
	(load_canonical_type_ids): The xml_reader namespace doesn't exist
	anymore.  It's now abixml.
	* tools/abidiff.cc (options::options): Initialize the
	do_debug_self_comparison data member before the use_btf one, as it
	should be.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>