The #includes <linux/bug.h> is here to use BUILD_BUG_ON_ZERO().

Thanks to commit bc6245e5 ("bug: split BUILD_BUG stuff out into
<linux/build_bug.h>"), it is now possible to reduce the number of
headers pulled in.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>