LINUX: Bring debug symbols back to the Linux kernel module.

LINUX: Bring debug symbols back to the Linux kernel module.

Starting with 4.8 Linux kernels our existing build script
generator, make_kbuild_makefile.pl, does not pass the debugging
symbols CFLAGS that were present when building for previous kernels.

This fix appends the $(KERN_DBG) variable which will only be defined
when the configuration includes the --enable-debug-kernel option.

Reviewed-on: https://gerrit.openafs.org/12519
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 961cee00b8f5c302de5f66beb81caa33242c7971)

Change-Id: I1d16382c4a744d4624cac9a9ba2810fa664abe93
Reviewed-on: https://gerrit.openafs.org/12534
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>