SDKs: Add inttypes.h to the darwin SDK stub

This unbreaks the configure build on darwin, where we build with the
stub SDK in -isysroot. Oddly, the cmake build doesn't have this
restriction, which probably means the cmake configs are out of date.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@198676 91177308-0d34-0410-b5e6-96231b3b80d8