Fix macOS checkerboard helper declaration

src/legacy_canvas_draw_merge_services.h

@@ -65,6 +65,9 @@ struct LegacyCanvasDrawMergeShaderExecution {
     std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
 };
 
+inline void setup_legacy_canvas_draw_merge_checkerboard_shader(
+    const LegacyCanvasDrawMergeCheckerboardUniforms& uniforms);
+
 struct LegacyCanvasDrawMergeLayerBlendUniforms {
     LegacyCanvasDrawMergeTextureBlendUniforms shader;
     bool copy_destination = false;