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

The check-installed-headers tests show up that the SH <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 SH3 and SH4 (compilation only).

	* sysdeps/unix/sysv/linux/sh/sys/user.h: Include <stddef.h>.