filter: remove unused variable

[nokute78]

This patch is to remove following warning.

[ 67%] Building C object src/CMakeFiles/fluent-bit-shared.dir/flb_filter.c.o
/home/taka/git/fluent-bit/src/flb_filter.c: In function ‘is_active’:
/home/taka/git/fluent-bit/src/flb_filter.c:52:21: warning: unused variable ‘m_head’ [-Wunused-variable]
   52 |     struct mk_list *m_head;
      |                     ^~~~~~

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change

• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.