Replace an irrelevant include of bug.h with the more appropriate
includes of slab.h and module.h.

it's not as if the original inclusion is an error, it's simply not
related to the contents of that source file, while the other two are.

compile-tested on i386.
Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>