Add deprecation facilities

Add deprecation facilities

Add the macro 'ABG_DEPRECATED' to mark APIs as to be removed in a next
major release. APIs marked with that flag are supposed to work as
before, but might come with downsides. E.g. they could perform worse or
provide only limited functionality. All deprecated functions shall come
with a hint to equivalent functionality within the non-deprecated part
of the API.

	* include/abg-fwd.h: Introduce deprecation macro ABG_DEPRECATED

Signed-off-by: Matthias Maennich <maennich@google.com>