kmod-sched-core: Add HTB and TBF traffic shapers

HTB and TBF are the basic traffic shapers used by sqm-scripts. Moving
these into kmod-sched-core enables sqm-scripts to downgrade its
dependency from kmod-sched to kmod-sched-core, potentially making it
useful on devices with smaller flash sizes.

This adds around 30k to the size of kmod-sched-core (20k for sch_htb.ko
and 10k for sch_tbf.ko).

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>