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/core/test/test_dcb.cc

@@ -36,8 +36,8 @@
 
 #include <maxscale/config.h>
 #include <maxscale/listener.h>
+#include <maxscale/routingworker.hh>
 
-#include "../internal/routingworker.hh"
 #include "../dcb.cc"
 #include "test_utils.h"