selftests/mm: add a few missing gitignore files

selftests/mm: add a few missing gitignore files

Compiled binary files should be added to .gitignore
'git status' complains:
   Untracked files:
   (use "git add <file>..." to include in what will be committed)
         mm/hugetlb_dio
         mm/pkey_sighandler_tests_32
         mm/pkey_sighandler_tests_64

Link: https://lkml.kernel.org/r/20241125064036.413536-1-lizhijian@fujitsu.com
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Donet Tom <donettom@linux.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>