gccrs: ast: Fix tokenstream extern block statements

gccrs: ast: Fix tokenstream extern block statements

The visitor created some additional semicolon between statements in
extern blocks, this lead to empty statement.

gcc/rust/ChangeLog:

	* ast/rust-ast-tokenstream.cc (TokenStream::visit): Remove
	additional semicolon output.
	* ast/rust-ast-tokenstream.h: Change block visitor prototype
	with a default value for trailing tokens.

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