The check-installed-headers tests show up that the MIPS <sys/user.h>
is not self-contained, using size_t without including any header that
defines it. This patch fixes it by including <stddef.h>, as done for
other architectures' versions of this header.
Tested for MIPS (all 24 ABIs, compilation only).
* sysdeps/unix/sysv/linux/mips/sys/user.h: Include <stddef.h>.