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

The check-installed-headers tests show up that the Alpha <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 Alpha (compilation only).

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

ChangeLog

@@ -1,3 +1,7 @@
+2016-11-17  Joseph Myers  <joseph@codesourcery.com>
+
+	* sysdeps/unix/sysv/linux/alpha/sys/user.h: Include <stddef.h>.
+
 2016-11-16  Joseph Myers  <joseph@codesourcery.com>
 
 	* conform/Makefile (linknamespace-libs): Rename to

sysdeps/unix/sysv/linux/alpha/sys/user.h

@@ -23,6 +23,7 @@
    than gdb/strace unless you know what you are doing. */
 
 #include <asm/reg.h>
+#include <stddef.h>
 
 struct user
 {