selftests: netfilter: Add missing gitignore file

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

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>