livepatch/klp-build: Fix klp-build vs CONFIG_MODULE_SRCVERSION_ALL

livepatch/klp-build: Fix klp-build vs CONFIG_MODULE_SRCVERSION_ALL

When building a patch to a single-file kernel module with
CONFIG_MODULE_SRCVERSION_ALL enabled, the klp-build module link fails in
modpost:

  Diffing objects
  drivers/md/raid0.o: changed function: raid0_run
  Building patch module: livepatch-0001-patch-raid0_run.ko
  drivers/md/raid0.c: No such file or directory
  ...

The problem here is that klp-build copied drivers/md/.raid0.o.cmd to the
module build directory, but it didn't also copy over the input source
file listed in the .cmd file:

  source_drivers/md/raid0.o := drivers/md/raid0.c

So modpost dies due to the missing .c file which is needed for
calculating checksums for CONFIG_MODULE_SRCVERSION_ALL.

Instead of copying the original .cmd file, just create an empty one.
Modpost only requires that it exists.  The original object's build
dependencies are irrelevant for the frankenobjects used by klp-build.

Fixes: 24ebfcd65a87 ("livepatch/klp-build: Introduce klp-build script for generating livepatch modules")
Reported-by: Song Liu <song@kernel.org>
Tested-by: Song Liu <song@kernel.org>
Link: https://patch.msgid.link/c41b6629e02775e4c1015259aa36065b3fe2f0f3.1769471792.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>