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

Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/x86/um/os-Linux/registers.c b/arch/x86/um/os-Linux/registers.c
index fa2e5a6..0cdbb86 100644
--- a/arch/x86/um/os-Linux/registers.c
+++ b/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"