clang-format: Fix header ordering.

Fix a bug in clang-format that causes STL headers that start with a 't' being grouped with Root headers. (Affects for example <type_traits>.) This is caused by regex matching in clang-format being case insensitve by default. The option for case sensitive matching is only introduced with clang-format 12. So as a workaround for clang-format 11 also match a '.h'-suffix for the filenames in these categories. This ensures seperation between Root headers and STL headers.