force arches to provide sys/user.h

Now that Linux is no longer installing linux/user.h, arches will have to
provide their own.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

libc/sysdeps/linux/common/sys/user.h

@@ -1 +1 @@
-#include <linux/user.h>
+#error "This file is machine-dependent and not provided for this machine."