[libc] Define 'size_t' in `time.h` and `uchar.h` (#103441)
Summary:
The standard says the following: The <time.h> header shall define the
clock_t, size_t, time_t, types as described in <sys/types.h>. I couldn't
find one for `uchar.h` but it needs it once we define things like
`mbrtoc8`.
