Fix -Watimport-in-framework-header

[qmfrederik]

As per https://developer.apple.com/documentation/xcode_release_notes/xcode_11_4_release_notes?language=objc

Fixes #722

[robbiehanson]

From Apple's Release Notes:

Xcode 11.4 turns the -Watimport-in-framework-header warning on by default. Using @import in framework headers is discouraged, because doing so requires all importers to use modules.

So is Apple saying we should be using this ??? :

#import <Foundation/Foundation.h>

[mykola-mokhnach]

this is the original llvm PR: https://reviews.llvm.org/rC335780