deps: turn in std::string for ICU

- node and v8 did not call into std::string previously,
so that access was shut off.
- this fixes compilation for ICU 58.2 (backlevel) but may
be expressed in other versions also.

Fixes: https://github.com/nodejs/node/issues/19151

PR-URL: https://github.com/nodejs/node/pull/19624


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>