Make MIPS <sys/user.h> self-contained.

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>.