Clean up timespec workaround

There's currently a timespec definition in riot-all.h:

// while GCC invoked by RIOT finds the newlib definitions, CLANG does not.
//
// This workaround fixes the fallout (that each time a *timespec is used, a
// `struct timespec` is generated by bindgen); actual solutions might be
// passing on the CC from the RIOT build system.
struct timespec { char *_unknown;};

This should at very least be made to work with native again, but preferably be fixed at its root.