<inttypes.h> can't be used in dynamic framework builds for iOS (under CocoaPods)
os_types.h on Darwin-based macOS/iOS/etc #includes <inttypes.h> to get at some system types. This fails on current iOS SDKs when building as a dynamic framework under the CocoaPods package manager, which to be Xcode-friendly needs to use "modular" header stuff.
<sys/types.h> provides the same stuff but works in the modular context. It's conceivable there's a back-compat issue with older SDKs, which I can't easily test right now though.