selftests/mm: add hugetlb_fault_after_madv to .gitignore

commit 116d5730 ("selftests/mm: add a new test for madv and hugetlb")
added a new test case, but, it didn't add the binary name in
tools/testing/selftests/mm/.gitignore.

Add hugetlb_fault_after_madv to tools/testing/selftests/mm/.gitignore.

Link: https://lkml.kernel.org/r/20231103173400.1608403-2-leitao@debian.org
Fixes: 116d5730

 ("selftests/mm: add a new test for madv and hugetlb")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/all/662df57e-47f1-4c15-9b84-f2f2d587fc5c@arm.com/
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>