Flip macCatalyst check for #704

[vincentjames501]

Fixes #704

Description

See #704 for details

Steps to verify

It compiles now on Catalyst :)

[gispada]

Good catch, I actually didn't try to compile on Catalyst.

On second thought, it seems that for older SDKs TARGET_OS_MACCATALYST and TARGET_OS_IOS are separate, while more recently TARGET_OS_MACCATALYST is inside TARGET_OS_IOS.

Would it be safer to do something like this? Does it work for you?

#if (TARGET_OS_IOS && TARGET_OS_MACCATALYST) || TARGET_OS_MACCATALYST
    externalUserAgent = [[OIDExternalUserAgentCatalyst alloc] initWithPresentingViewController:presentingViewController];
#elif TARGET_OS_IOS
    externalUserAgent = [[OIDExternalUserAgentIOS alloc] initWithPresentingViewController:presentingViewController];
#elif TARGET_OS_OSX
    externalUserAgent = [[OIDExternalUserAgentMac alloc] init];
#endif