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

[perillo]

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.