Emit an error on variadic non extern functions

Emit an error on variadic non extern functions

Variadic regular functions were recently added in the parser as they
should be rejected in the ast validation pass. This commit add the ast
validation pass rejecting this kind of variadic arguments.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Add
	ast validation pass to reject variadic arguments on regular functions.

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