selftests: add .gitignore for kcmp

Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/kcmp/.gitignore

+kcmp_test
+kcmp-test-file