cube: Set pipeline renderpass

LunarXchange issue #111:
https://vulkan.lunarg.com/app/issues/55cbc6d77ef24d0001000001
noticed that the pipeline's renaderPass was not being set.
diff --git a/demos/cube.c b/demos/cube.c
index 0d0dff2..662f31c 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -1647,6 +1647,8 @@
     pipeline.pStages             = shaderStages;
     pipeline.renderPass          = demo->render_pass;
 
+    pipeline.renderPass = demo->render_pass;
+
     err = vkCreateGraphicsPipelines(demo->device, demo->pipelineCache, 1, &pipeline, &demo->pipeline);
     assert(!err);