um: we need sys/user.h only on i386

Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/os-Linux/registers.c

@@ -6,7 +6,9 @@
 
 #include <errno.h>
 #include <sys/ptrace.h>
+#ifdef __i386__
 #include <sys/user.h>
+#endif
 #include "longjmp.h"
 #include "sysdep/ptrace_user.h"