nix: deploy WK2 header WKPageVisibilityTypes.h

Since recent commit 5fd52db0 (Improve PageVisibility API with enums
https://bugs.webkit.org/show_bug.cgi?id=107364), WKPageVisibilityTypes.h
- from the shared C API - is included by WebKit2/UIProcess/API/C/WKPage.h.

WKPageVisibilityTypes.h wasn't being installed and hence it was causing
problems for applications using WKPage.h.

Reviewed by Jesus Sanchez-Palencia