datapath: bug.h missing from distfiles

commit 7c359202 introduced datapath/linux/compat/include/bug.h
but did not include it in datapath/linux/Modules.mk, which results
in the following build error:

> The distribution is missing the following files:
> datapath/linux/compat/include/linux/bug.h

Signed-off-by: Chris Luke <chris_luke@cable.comcast.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>