selftest/lkdtm: Use local .gitignore

Commit 68ca0fd2 ("selftest/lkdtm: Don't pollute 'git status'")
introduced patterns for git to ignore files generated in
tools/testing/selftests/lkdtm/

Use local .gitignore file instead of using the root one.

Fixes: 68ca0fd2

 ("selftest/lkdtm: Don't pollute 'git status'")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>