OSMC is structured a little differently from Raspbian. For example, the headers package for the Pi2/3 is named rbp2-headers-$(uname -r).
Second, the header file location is different, so you’ll need to add a link from the location expected by DKMS, eg:
sudo ln -s /usr/src/rbp2-headers-$(uname -r) /lib/modules/$(uname -r)/build