debian: Allow kmod or module-init-tools for backward compatibility.

Commit d473844693 (debian: Depend on 'kmod' instead of module-init-tools.)
switched from depending on module-init-tools to depending on kmod, which
is the new name of the appropriate package in Debian.  Unfortunately,
while kmod is the right name for the latest Debian distribution, it doesn't
have that name in old distributions and thus breaks the build on those.
This commit should work OK in either case, since it allows both names.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>