Support iOS11

[fofewyoung]

i tried to build static library for iOS, and found it support iOS13 only , i expect it support iOS11 (in Unity Games)
so i changed DEPLOYMENT_TARGET to 11.0, and run the build script again, it show:
platform_posix.c:450:5: error: 'timespec_get' is only available on iOS 13.0 or newer
certificates_darwin.c:152:14: error: 'SecTrustEvaluateWithError' is only available on iOS 12.0 or newer
will msquic support iOS11?

[nibanks]

If you have possible fixes, please propose a PR. We don't have a lot of experience with iOS ourselves.

[fofewyoung]

i proposed a PR:
https://github.com/microsoft/msquic/pull/2898/files