In https://svnweb.freebsd.org/changeset/base/288176 we changed debug symbols
file from .symbols to .debug, this also affected the ports installs debug kmods.

This patch determines the correct suffix by OSREL.

I think later this might be integrated into kmod.mk and we might also need to
install debug symbols file of kmod from ports to /usr/lib/debug/boot/modules .

I'll prepare another patch about this later, in the meantime, can we have this
fix and make me can continue on my other work with virtualbox?

Thanks.