Provide our own <timespec.h> and <_timespec.h> headers

Provide our own <timespec.h> and <_timespec.h> headers

Newlib uses <timespec.h> and <_timespec.h> headers for defining itimerspec and
timespec types, respectively. However, these definitions are not protected with
preprocessing guards in case they were previously defined, which happens for us
because we define these types in uktime internal lib. Therefore the solution is
to skip using newlib's headers and provide our own.

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>