compiler: header file generated by `zig build-lib -femit-h` includes zig.h

I'm trying to make the example in https://ziglang.org/documentation/0.10.1/#toc-Exporting-a-C-Library work.

`zig build-obj -femit-h mathtest.zig` correctly emits the `mathtest.h` header file, but the problem is that the file includes `zig.h` so that `zig build` still fails since `zig.h` is not found.

See PR #14378.