Use struct when declaring structs in x3::rule

It doesn't matter if you use struct or class, but the last patch used
struct, so let's make it that way everywhere else.

Change-Id: I17191789c8b35e2d12f922d559e76e02d40c3780
diff --git a/src/grammars.hpp b/src/grammars.hpp
index 540201c..51361fc 100644
--- a/src/grammars.hpp
+++ b/src/grammars.hpp
@@ -141,8 +141,8 @@
     }
 } const datastore;
 
-const auto copy_source = x3::rule<class source, Datastore>{"source datastore"} = datastore;
-const auto copy_destination = x3::rule<class source, Datastore>{"destination datastore"} = datastore;
+const auto copy_source = x3::rule<struct source, Datastore>{"source datastore"} = datastore;
+const auto copy_destination = x3::rule<struct source, Datastore>{"destination datastore"} = datastore;
 
 const auto datastoreSuggestions = staticSuggestions({"running", "startup"});