Update 15-ios.conf

Update 15-ios.conf

CLA: trivial

I assume this has been an error in the initial ios conf file. In order to build for ios the shared engine library, needs to be disabled because iOS doesn't have the concept of shared libraries. But instead of only disabling `dynamic-engine` (or like in this commit disabled the `shared`) option the previous config did disable `engine` and with that the `static-engine` compilation as well. This restores the `static-engine` option being enabled by default, but keeping compilation going on iOS.

Cheers!

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/16659)

(cherry picked from commit aa58071e4b8b245db1564f476731c978738e7e98)