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

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

while here, sort the headers.

Fixes: 757e973fb211 ("libc tests: add tests for link_addr(3) and link_ntoa(3)")
Reviewed by: des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50189