Fix missing include of string.h in apps/lib/engine.c for strcmp.
Fix missing include of string.h in apps/lib/engine.c for strcmp.

This include is required for c99 on the NonStop TNS/X platform.

CLA: trivial

Fixes #13102

Signed-off-by: Randall S. Becker <rsbecker@nexbridge.com>

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13103)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl