It's used in sixlowpan's `flowcontrol.[ch]`. One test that uses this module is `test_pnet`, which is far out of my scope. `flowcontrol.h` needs `sys/types.h` and/or `stddef.h` because it uses `size_t`. At this point we have a nicely reduced the problem to the original problem of this PR, because `flowcontrol.h` uses semaphores, too. Either way the test builds for non-msp430 boards even without locally including `sys/types.h` in `flowcontrol.h`.