[build.zig] Fix raygui build when using addRaygui externally

When addRaygui is used from an external build, for example in a
bindings project, the build of a generated `raygui.c` fails with
"raylib.h not found" error from the compiler.

I've traced this down to a raylib step not adding its `src/` to the
shared list of include paths using `addIncludePath` but relying on
`addCSourceFiles` `.root` to provide the implicit include path for
raylib proper's own files.

If raygui is later added to the step the compiler won't know where
to look for `raylib.h` and friends and will fail to build.

This change simply adds raylib's `src/` to the include path list.

src/build.zig

@@ -216,6 +216,7 @@ fn compileRaylib(b: *std.Build, target: std.Build.ResolvedTarget, optimize: std.
         },
     }
 
+    raylib.addIncludePath(b.path("src"));
     raylib.root_module.addCSourceFiles(.{
         .root = b.path("src"),
         .files = c_source_files.items,