add raygui

zig/build.zig

@@ -20,6 +20,7 @@ pub fn build(b: *std.Build) void {
         .target = target,
         .optimize = optimize,
     });
+    exe.addIncludePath(.{ .path = "3rd-party/raygui/src" });
     exe.linkLibrary(raylib_dep.artifact("raylib"));
 
     // This declares intent for the executable to be installed into the