aarch64: fix undefined "struct user_regs_struct_t" error

tested under Gentoo Linux, with sys-kernel/linux-headers-3.19 installed
"struct user_pt_regs" is defined at file /usr/include/asm/ptrace.h
Signed-off-by: Yixun Lan <yixun.lan@gmail.com>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

arch/aarch64/include/asm/types.h

@@ -3,6 +3,7 @@
 
 #include <stdbool.h>
 #include <signal.h>
+#include <asm/ptrace.h>
 #include "protobuf/core.pb-c.h"
 
 #include "asm-generic/page.h"