KVM: selftests: Explicitly include committed one-off assets in .gitignore

Add KVM selftests' one-off assets, e.g. the Makefile, to the .gitignore so
that they are explicitly included. The justification for omitting the
one-offs was that including them wouldn't help prevent mistakes:

Deliberately do not include the one-off assets, e.g. config, settings,
.gitignore itself, etc as Git doesn't ignore files that are already in
the repository. Adding the one-off assets won't prevent mistakes where
developers forget to --force add files that don't match the "allowed".

Turns out that's not the case, as W=1 will generate warnings, and the
amazing-as-always kernel test bot reports new warnings:

tools/testing/selftests/kvm/.gitignore: warning: ignored by one of the .gitignore files
tools/testing/selftests/kvm/Makefile: warning: ignored by one of the .gitignore files
>> tools/testing/selftests/kvm/Makefile.kvm: warning: ignored by one of the .gitignore files
tools/testing/selftests/kvm/config: warning: ignored by one of the .gitignore files
tools/testing/selftests/kvm/settings: warning: ignored by one of the .gitignore files

Fixes: 43e96957e8b8 ("KVM: selftests: Use pattern matching in .gitignore")
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

tools/testing/selftests/kvm/.gitignore

···
 !*.h
 !*.S
 !*.sh
+!.gitignore
+!config
+!settings
+!Makefile