MXS-1840 Compile all routers as C++

Minimal changes, only what is needed in order to make it compile.

server/modules/routing/CMakeLists.txt

@@ -5,6 +5,7 @@ if(BUILD_BINLOG)
   add_subdirectory(binlogrouter)
 endif()
 
+add_subdirectory(cat)
 add_subdirectory(cli)
 add_subdirectory(debugcli)
 add_subdirectory(hintrouter)
@@ -12,6 +13,5 @@ add_subdirectory(maxinfo)
 add_subdirectory(readconnroute)
 add_subdirectory(readwritesplit)
 add_subdirectory(schemarouter)
-add_subdirectory(cat)