FBSD: bsd.kmod.mk fallout for debug symbols

FBSD: bsd.kmod.mk fallout for debug symbols

Now that we no longer control the installation of libafs.ko,
debug symbols are always getting stripped.
If the user requested kernel debug symbols, define DEBUG_FLAGS
for the libafs build and install, so that bsd.kmod.mk will
install the .symbols file alongside the kernel module.

Change-Id: I0e7f3736666c96c6aaed18c0add98dea37349b83
Reviewed-on: http://gerrit.openafs.org/5503
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>