Added 'fallthrough' comments to suppress gcc7 warnings.

erb/v1/cpp03_define_array.hpp.erb

@@ -66,7 +66,9 @@ struct define_array<A0<%1.upto(i) {|j|%>, A<%=j%><%}%>> {
             msgpack::object *ptr = o.via.array.ptr;
             switch(size) {
             default:<%(i).downto(0) {|j|%>
-            case <%=j+1%>: ptr[<%=j%>].convert(a<%=j%>);<%}%>
+            case <%=j+1%>: ptr[<%=j%>].convert(a<%=j%>);
+            // fallthrough
+<%}%>
             }
         }
     }