Skip inttypes.h on Darwin

Skip inttypes.h on Darwin

When building a dynamic framework with Swift compatibility, can't
include any headers that aren't in another module or you get an
error like this from Xcode on the including project:

  Include of non-modular header inside framework

For some reason the system inttypes.h is not in a module, unlike
other standard C library headers... but it doesn't seem to be
actually needed on Darwin, so removing it doesn't appear to
be a problem.

Change-Id: I11d264483c54feefd9d2edf573afaef34ddcd0f2