LibGfx+LibWeb: Fix compile errors in clang-cl from recent header cleanup

[InvalidUsernameException]

The recent commits 28ba610f325aaed5d20daebdd3cf732a04e62eaf and 70c4ed261fb446fe25396164400d09e0c0df8bc6 adjusted some include directives to avoid excessive recompilation when changing some header files. This has broken compilation with clang-cl on Windows without getting noticed before the PRs were merged.

[nico]

I'm guessing that's because WEB_API forces emission of inlines. If you aren't already, you could consider using https://blog.llvm.org/2018/11/30-faster-windows-builds-with-clang-cl_14.html to make the win build more like the other platforms (makes the build faster, too).

[InvalidUsernameException]

Looks like that flag is already used:

https://github.com/LadybirdBrowser/ladybird/blob/c1c24e8fd6fd3302d164e4198458d837523b2695/Meta/CMake/common_compile_options.cmake#L97