Make routingworker.hh public

Given that worker.hh was public, it made sense to make routingworker.hh
public as well. This removes the need to include private headers in
modules and allows C++ constructs to be used in C++ code when previously
only the C API was available.

server/modules/protocol/MySQL/mariadb_client.cc

@@ -12,11 +12,9 @@
  */
 
 #include <maxscale/protocol/mariadb_client.hh>
+#include <maxscale/routingworker.hh>
 #include <maxscale/utils.h>
 
-// TODO: Find a way to cleanly expose this
-#include "../../../core/internal/routingworker.hh"
-
 #ifdef EPOLLRDHUP
 #define ERROR_EVENTS (EPOLLRDHUP | EPOLLHUP | EPOLLERR)
 #else