linux/types.h: Surround 'struct ustat' with __linux__

'struct ustat' uses linux-specific typedefs to declare its memebers:
__kernel_daddr_t and __kernel_ino_t. It is currently not used by any
U-Boot codes, but when we build U-Boot tools for other platform like
Windows, this becomes a problem.

Let's surround it with __linux__.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>