[hsflowd] Fix .gitignore to not ignore patch/

[bluecmd]

Why I did it

If you added a patch in the patch/ subdir it would be ignored by default.

How I did it

Modify .gitignore

How to verify it

Add file in patch/ and see that it is now not ignored in git status.

E.g.:

$ git status 
On branch 202211
Your branch is up to date with 'origin/202211'.

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
        modified:   series

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   ../.gitignore

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        9999-kamel_jumbo-frames.patch
        dropmon/asdasd

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

N/A

Link to config_db schema for YANG module changes

N/A

A picture of a cute animal (not mandatory but encouraged)