aspeed/timer: fix compile breakage with clang 3.4.2

aspeed/timer: fix compile breakage with clang 3.4.2

In file included from /home/thuth/devel/qemu/hw/timer/aspeed_timer.c:16:
/home/thuth/devel/qemu/include/hw/misc/aspeed_scu.h:37:3: error:
redefinition of typedef 'AspeedSCUState' is a C11 feature
      [-Werror,-Wtypedef-redefinition]
} AspeedSCUState;
  ^
/home/thuth/devel/qemu/include/hw/timer/aspeed_timer.h:27:31: note:
previous definition is here
typedef struct AspeedSCUState AspeedSCUState;

Reported-by: Thomas Huth <[email protected]>
Signed-off-by: Cédric Le Goater <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20180921161939[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>