src: add missing #include <unicode/ustring.h>

* We use these functions that are declared in <unicode/ustring.h>

  u_strFromUTF8()
    u_strToUTF8()

* At present, <unicode/ustring.h> is indirectly included, but this will
likely change in future ICUs. Adding this header has been the right
thing to do for many years, so it is backwards compatible.

Fixes: https://github.com/nodejs/node/issues/11753
PR-URL: https://github.com/nodejs/node/issues/11754


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>