st/mesa: change glsl_to_tgsi_visitor from class to struct

To match the declaration in the .h file and silence an MSVC warning.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -285,7 +285,7 @@ public:
    st_src_reg return_reg;
 };
 
-class glsl_to_tgsi_visitor : public ir_visitor {
+struct glsl_to_tgsi_visitor : public ir_visitor {
 public:
    glsl_to_tgsi_visitor();
    ~glsl_to_tgsi_visitor();