kbuild: Do not run kernel-doc when building external modules

After commit 778b8ebe ("docs: Move the python libraries to
tools/lib/python"), building an external module with any value of W=
against the output of install-extmod-build fails with:

  $ make -C /usr/lib/modules/6.19.0-rc7-00108-g4d310797262f/build M=$PWD W=1
  make: Entering directory '/usr/lib/modules/6.19.0-rc7-00108-g4d310797262f/build'
  make[1]: Entering directory '...'
    CC [M] ...
  Traceback (most recent call last):
    File "/usr/lib/modules/6.19.0-rc7-00108-g4d310797262f/build/scripts/kernel-doc.py", line 339, in <module>
      main()
      ~~~~^^
    File "/usr/lib/modules/6.19.0-rc7-00108-g4d310797262f/build/scripts/kernel-doc.py", line 295, in main
      from kdoc.kdoc_files import KernelFiles             # pylint: disable=C0415
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ModuleNotFoundError: No module named 'kdoc'

scripts/lib was included in the build directory from find_in_scripts but
after the move to tools/lib/python, it is no longer included, breaking
kernel-doc.py.

Commit eba6ffd1 ("docs: kdoc: move kernel-doc to tools/docs") breaks
this even further by moving kernel-doc outside of scripts as well, so it
cannot be found when called by cmd_checkdoc.

  $ make -C /usr/lib/modules/6.19.0-rc7-next-20260130/build M=$PWD W=1
  make: Entering directory '/usr/lib/modules/6.19.0-rc7-next-20260130/build'
  make[1]: Entering directory '...'
    CC [M]  ...
  python3: can't open file '/usr/lib/modules/6.19.0-rc7-next-20260130/build/tools/docs/kernel-doc': [Errno 2] No such file or directory

While kernel-doc could be useful for external modules, it is more useful
for in-tree documentation that will be build and included in htmldocs.
Rather than including it in install-extmod-build, just skip running
kernel-doc for the external module build.

Cc: stable@vger.kernel.org
Fixes: 778b8ebe ("docs: Move the python libraries to tools/lib/python")
Reported-by: Rong Zhang <i@rong.moe>
Closes: https://lore.kernel.org/20260129175321.415295-1-i@rong.moe/


Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260130-kbuild-skip-kernel-doc-extmod-v1-1-58443d60131a@kernel.org


Reviewed-by: Nicolas Schier <nsc@kernel.org>
Signed-off-by: Nicolas Schier <nsc@kernel.org>