Sets 'Require Only App-Extension-Safe API' build settings to YES on t…

[umdemir]

…argets (Promises and FBLPromises) to clear out the 'ld: warning: linking against a dylib which is not safe for use in application extensions' warning

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[umdemir]

I've signed the CLA.

[googlebot]

CLAs look good, thanks!

[shoumikhin]

Thank you, Umit! Curious why you preferred setting that flag for the two framework targets only, but not for the whole project, so that all other targets inherit it?

[umdemir]

I didn't want to touch the test projects because I assumed they're not going to run in a app extension or a real device. Actually it probably wouldn't hurt to set it on the project level. That way new targets would also inherit it without any changes. I think I'd like to set it on the project level now if it's ok with you too :) Thanks for the question!