link_addr_test: use <cstddef>, not <sys/stddef.h>

[ivy]

Summary

<cstddef> is the correct header; this fixes the GCC build.

Fixes: 757e973fb211 ("libc tests: add tests for link_addr(3) and link_ntoa(3)")

[des]

lib/libc/tests/net/link_addr_test.cc

23–24

36

any reason not to sort these?

[ivy]

sort the headers

i've separated the C headers from the C++ headers and sorted them separately; i
don't know if we have any existing style guidelines for that but it's how i
usually prefer to do it, otherwise you end up with a bunch of C headers
randomly sorted under 'c'.

[des]

I would have preferred not to split them, but no strong opinion either way.