xtensa: fix build error in xtensa/include/asm/io.h

Caused by commit 6c03438e

    kernel.h: doesn't explicitly use bug.h, so don't include it.

This header uses bug.h so explicitly include it now that the
implicit presence was removed by 6c03438e.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>