x86: Fix i8259 ifdef include guard

When building U-Boot with clang, it notices that the i8259.h include
guard does not work correctly due to a typo. Fix it.

Signed-off-by: Alistair Delva <adelva@google.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>