[mojo] Add GenericInterface_ to interface class variants

mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl

Index: mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
index 536a662ac1c47c12544b3827c3d0d977628150df..2aff6bd9770bba0501b0c5873caf9256e4ada338 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/module.h.tmpl
@@ -34,6 +34,9 @@
 {%- for import in imports %}
 #include "{{import.module.path}}.h"
 {%- endfor %}
+{%- if variant %}
+#include "{{module.path}}.h"
+{%- endif %}
 
 {%- for namespace in namespaces_as_array %}
 namespace {{namespace}} {