qapi: Record 'include' directives in parse tree

qapi: Record 'include' directives in parse tree

The parse tree is a list of expressions.  Except include expressions
currently get replaced by the included file's parse tree.

Instead of throwing away the include expression, keep it with the file
name expanded so you don't have to track the including file's
directory to make sense of it.

A future commit will put this include expression to use.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180211093607.27351-16-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: fix check of expr after assignment]
Signed-off-by: Eric Blake <eblake@redhat.com>