Support exporting Mojo bindings for the component build.

mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl

 {% from "enum_macros.tmpl" import enum_decl -%}
 
-class {{struct.name}} {
+class {{export_attribute}} {{struct.name}} {
  public:
   using DataView = {{struct.name}}DataView;
   using Data_ = internal::{{struct.name}}_Data;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
 {%-   if enum|is_native_only_kind %}
   using {{enum.name}} = mojo::NativeEnum;
 {%-   else %}
   {{enum_decl(enum)|indent(2)}}
@@ skipping 56 matching lines @@
   }
 
 {#--- Struct members #}
 {%  for field in struct.fields %}
 {%-   set type = field.kind|cpp_wrapper_type %}
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 };