iris: Move iris_stream_output_target def to iris_context.h

now that it doesn't have genxml

src/gallium/drivers/iris/iris_context.h

@@ -306,6 +306,16 @@ struct iris_shader_state {
    uint32_t bound_sampler_views;
 };
 
+/**
+ * Gallium CSO for stream output (transform feedback) targets.
+ */
+struct iris_stream_output_target {
+   struct pipe_stream_output_target base;
+
+   /** Storage holding the offset where we're writing in the buffer */
+   struct iris_state_ref offset;
+};
+
 /**
  * Virtual table for generation-specific (genxml) function calls.
  */