vfio: selftests: Fix .gitignore for already tracked files

vfio: selftests: Fix .gitignore for already tracked files

Fix the rules in tools/testing/selftests/vfio/.gitignore to not ignore
some already tracked files (.gitignore, Makefile, lib/libvfio.mk).

This change should be a no-op, since these files are already tracked by git
and thus git will not ignore updates to them even though they match the
ignore rules in the VFIO selftests .gitignore file.

However, they do generate warnings with W=1, as reported by the kernel test
robot.

  $ KBUILD_EXTRA_WARN=1 scripts/misc-check
  tools/testing/selftests/vfio/.gitignore: warning: ignored by one of the .gitignore files
  tools/testing/selftests/vfio/Makefile: warning: ignored by one of the .gitignore files
  tools/testing/selftests/vfio/lib/libvfio.mk: warning: ignored by one of the .gitignore files

Fix this by explicitly un-ignoring the tracked files.

Fixes: 292e9ee22b0a ("selftests: Create tools/testing/selftests/vfio")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508280918.rFRyiLEU-lkp@intel.com/
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250828185815.382215-1-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>