clang-format header include grouping: _class.h has top priority

In some cases, the main include file has a suffix _class.h, and should
therefore be placed at the top. This is a follow-up to 3b7b19a.

Author: tautschnig

.clang-format

@@ -49,6 +49,8 @@ PointerAlignment: Right
 ReflowComments: 'false'
 IncludeBlocks: Regroup
 IncludeCategories:
+  - Regex: '_class.h(>|")$'
+    Priority: 0
   - Regex: '<util/'
     Priority: 1
   - Regex: '<goto-programs/'