Escape windows specific paths generated by os.path.join so that it works with regex.

Note that this will work on all platforms but is critical for windows due to the use backslash in the filesystem.

Bug: webrtc:13607
Change-Id: Ie9a9987f1382133792c85820d38b770fadc0fff5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251442
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Christoffer Jansson <jansson@google.com>
Cr-Commit-Position: refs/heads/main@{#35982}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index acc3110..ed4e141 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -1231,9 +1231,8 @@
   # eval-ed and thus doesn't have __file__.
   error_msg = """{} should be listed in {}."""
   results = []
-  exempt_paths = [
-      os.path.join('tools_webrtc', 'ios', 'SDK'),
-  ]
+  exempt_paths = [re.escape(os.path.join('tools_webrtc', 'ios', 'SDK'))]
+
   with _AddToPath(
       input_api.os_path.join(input_api.PresubmitLocalPath(), 'tools_webrtc',
                              'presubmit_checks_lib')):