turnip: Silence compiler warning about uninit pipeline.

The code was fine as far as I see, but the warning was irritating.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>

src/freedreno/vulkan/tu_pipeline.c

@@ -1882,7 +1882,7 @@ tu_CreateGraphicsPipelines(VkDevice device,
       tu_pipeline_builder_init_graphics(&builder, dev, cache,
                                         &pCreateInfos[i], pAllocator);
 
-      struct tu_pipeline *pipeline;
+      struct tu_pipeline *pipeline = NULL;
       VkResult result = tu_pipeline_builder_build(&builder, &pipeline);
       tu_pipeline_builder_finish(&builder);