gcc-changelog: Add libffi/ to ignored_prefixes

gcc-changelog: Add libffi/ to ignored_prefixes

Add libffi/ to ignored_prefixes for syncing with libffi upstream:

commit c095f8f2e6f26bfc2ff8e3276c6af23ab153f5ff
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Tue Aug 31 07:14:47 2021 -0700

    libffi: Sync with libffi 3.4.2

    Merged commit: f9ea41683444ebe11cfa45b05223899764df28fb

to avoid

remote: *** The following commit was rejected by your hooks.commit-extra-checker script (status: 1)
remote: *** commit: c095f8f2e6f26bfc2ff8e3276c6af23ab153f5ff
remote: *** ChangeLog format failed:
remote: *** ERR: cannot find a ChangeLog location in message
remote: ***
remote: *** Please see: https://gcc.gnu.org/codingconventions.html#ChangeLogs
remote: ***
remote: error: hook declined to update refs/heads/master

	* gcc-changelog/git_commit.py (ignored_prefixes): Add libffi/.