Structs are highlighted by Function Declaration

This is a regression on master. I see this on 4aced20a2145 but not on 3f87c468 (tag: v4.6.0-beta1).

Steps:

0. In Options, choose "Single Underline" for "Function Declaration".
1. Open higlightingmarks.cpp
2. Go to line 67.

Actual: struct ConversionFunction ...

Expected: struct ConversionFunction ...

Once fixed, we need a test for this so we don't regress here again.