Small fix for consistency between C and C++ highlighting. Related to
https://github.com/zed-industries/zed/issues/9461

Release Notes:

- Change syntax highlighting for preprocessing directive in C so it can
be configured with `keyword.directive` instead of being treated as other
`keyword`. The behavior should be like the C++ one now.

<img width="953" height="343" alt="圖片"
src="https://github.com/user-attachments/assets/6b45d2cc-323d-44ba-995f-d77996757669"
/>