gccrs: Set traits getter as member function

gccrs: Set traits getter as member function

This function will be used outside of the expand visitor, making it
easily accessible is therefore mandatory.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (Attribute::get_traits_to_derive): Add
	function as member function.
	* ast/rust-ast.h: Add prototype.
	* expand/rust-expand-visitor.cc (get_traits_to_derive): Remove
	function.
	(ExpandVisitor::expand_inner_stmts): Update function call.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>