Fix some clang compile problems

Fix some clang compile problems

	* include/abg-comp-filter.h (class filter_base): Declare this as a
	struct.
	* include/abg-comparison.h (class filtering::filter_base):
	Likewise.
	(struct diff_traversable_base): Declare this as a class.
	* include/abg-ir.h (function_decl::parameter): Declare this before
	using it.
	* src/abg-corpus.cc
	(corpus::priv::build_unreferenced_symbols_tables): Add missing
	parenthesis around assignment expressions inside conditional
	expressions.

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