util/lint: Clear syntax warnings in Python 3.12

Since Python 3.12, invalid escape sequences produce a SyntaxWarning;
in the future, they will produce SyntaxError.

Using raw strings clear out the warning.

Below the command used for checking the fix worked.

```
$ python3 util/lint/checkpatch_json.py
```

Link: https://docs.python.org/3.12/whatsnew/3.12.html#other-language-changes
Change-Id: I0177dc7f0d3013759879320afdb6ab548d356bc7
Signed-off-by: Ariel Otilibili <otilibil@eurecom.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/85771
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

util/lint/checkpatch_json.py

@@ -39,7 +39,7 @@ def parse_file(input_file):
             file_path = temp[1].split(":")[0]
             line_number = temp[1].split(":")[1]
             update_struct( file_path.strip(), msg_output, str(line_number)  )
-        elif re.search("^\d+:\Z",line) != "None" and line.startswith("#"):
+        elif re.search(r"^\d+:\Z",line) != "None" and line.startswith("#"):
             file_path="/COMMIT_MSG"
             line = line.replace('#', '')
             line_number = int(line.split(":")[0]) + 2