Ignore FIXMEs at lgtm.com

We're using these for writing down stuff which should be -- eventually

• fixed, or as a TODO list. I've been doing that for 13+ years, and I

definitely won't be doing a tree-wide sed to make lgtm.com happy.

Change-Id: I921cdb211846e190b0e40cc7f3969f637cd17e36
Sort-of-fixes: https://lgtm.com/projects/g/KDE/trojita/alerts/?mode=list&rule=cpp%2Ffixme-comment