st/mesa: fix struct vs. class compilation warning

glsl_to_tgsi_visitor is earlier defined as a class, not a struct.
Fixes MSVC warning.

NOTE: This is a candidate for the 8.0 branch.

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -5093,7 +5093,7 @@ st_link_shader(struct gl_context *ctx, struct gl_shader_program *prog)
 }
 
 void
-st_translate_stream_output_info(struct glsl_to_tgsi_visitor *glsl_to_tgsi,
+st_translate_stream_output_info(glsl_to_tgsi_visitor *glsl_to_tgsi,
                                 const GLuint outputMapping[],
                                 struct pipe_stream_output_info *so)
 {