i386/macOS support: include fixup header where needed

Closed Felix Paul Kühne requested to merge fkuehne/medialibrary:fix-i386-macOS-build into master

When compiling for i386 on macOS or for the 32bit i386 iPhone Simulator, thread_local is not available. This was correctly detected by the build script but since the Fixup header was not included, the replacement was not used leading to compilation failure.

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