Include command "dkms add" in Makefile

DKMS was complaining about missing module in tree:
**Error! DKMS tree does not contain: bbswitch-0.8**
This adds the *dkms add* command that makes everything smooth on my machine (using dkms 2.0.19).
Suggesting this in case other people run into the same issue - I suspect it depends on the dkms version, but I'm not sure.