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.

Change-Id: I9a85dc0311a3a706239bc9e471b2d7197ebe1946
Reviewed-on: https://gerrit.openafs.org/12519
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>