coroutines: No cleanups on goto statements.

Minor cleanup, this is statement not an expression, we do not
need to use finish_expr_stmt here.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/cp/ChangeLog:

	* coroutines.cc (await_statement_walker): Use build_stmt and
	add_stmt instead of build1 and finish_expr_stmt.