Add headers to artifact include tree

[pvande]

This should enable other C dependencies to properly link against the library by doing something like the following:

const other_lib = b.addStaticLibrary(.{...});
other_lib.addCSourceFile(.{...});

other_lib.linkLibrary(b.dependency("zglfw", .{}));
// Or…
other_lib.addIncludePath(b.dependency("zglfw", .{}).getEmittedIncludeTree());

(Caveat: novice Zig user. Please feel free to correct any misunderstandings you see.)